Yes. Stick with your bump down to BN. You are not ready for Novice. See below.
First of all, don't make excuses. You have already stated this is a problem area, and now it's the mud? No. It's a problem area. No big deal--it's not like it's a massive disaster, just something you're working on. Accept that, don't make excuses for it, and you will improve.
As for the 4', if you were confidently and correctly jumping 4' you wouldn't be asking these questions. Having cleared 4' does not make one ready for any specific level.
You are fine at Pre BN or probably BN on this horse. You have a lot of good things in your riding, and you have a cute horse that looks to be the sort to teach you a lot and give you quality miles. You treat the horse well, and are looking to learn. Don't overdo it and wreck your horse's or your own confidence.
Good choices. BN should be cantering the course in theory, but if you are not ready then trot. The fact that you are thinking this is a sign that you were not ready for N. Certainly pay attention to his conditioning. Has he been ridden at all lately? If not I would scratch. You shouldn't jump AT ALL if the horse hasn't been under saddle.
NO NO NO NO NO. Yes, events need to be well attended, but first and foremost they need to be safe and fun for all involved.
No. Go when you think you're ready. Sure, sometimes you are a bit off in your assessment and must choose to scratch or retire. Sure, sometimes you have a bad day or the horse is off or whatnot and you need to scratch or retire, even though you are ready. These should be RARE occurrences.
Okay, now to your riding. The biggest issue is that you are not using your leg. I noticed it in the first few moments of the vid. Your lower leg is not against the horse and doesn't DO anything. You can see this clearly at about 5:40. You are also in a slight chair seat. Your seat is good and quiet, but you aren't using it.
First, remember that the contact should be evenly distributed across the inside of your leg. Your lower leg needs to be ON the horse so that it CAN do something. Only then can it actually ask the horse to lift his back and step under himself and push forward. You need to get that sorted. In order to get your leg in the proper position, try a few exercises. First, try doing some two point. Notice how your leg slips back? That's where it should be all the time. Then try standing straight up with your crot.ch over the pommel as the horse walks and trots. You will need soft knees and serious leg strength to do it for any length. Again you will notice your lower leg sliding back. It is where it should be. Begin posting, keeping your lower leg back. And finally, the hardest but most effective of these exercises (do the others first!) is to do the alternating posting. Sit, stand, stand, sit, stand, stand. You are effectively switching diagonals by standing (not sitting) an extra beat. Then you switch right back the very next stride. It's tougher than it sounds! Once your leg is in place, THEN you can begin to learn to use it properly!
You will also notice how much core you use. Your posture is already good, but you will find as you use your core more, it will become even better. Your core is your support to steady your upper torso and to help you follow your horse's motion. You will use your seat more effectively once your core is strong.
In the meantime, the other big things to work on include geometry, left lead, overall balance, and bend.
You can always, always be accurate. You are not going fully to the rail, not going into your corners, not precise in your locations and shapes--these are easy fixes. The next is the bend. You want the horse's ribs bent around your inside leg and his topline to follow the shape of your track. So if you are on a circle you want the horse's spine to be lightly and evenly curved to the arc of the circle. This means you should barely be able to see the corner of the inside eye, but it also means the horse's ribs are to the outside. Remember that the head should be in line with the shoulders! This helps the horse balance, and it is a major reason for the struggles with the left lead.
Again, you have a solid seat and good posture, but your hands are a tad unsteady (very evident at the 4:00 mark). As your seat becomes more independent and your base and core stronger, this will take care of itself. However, you need to work on how to approach a fence. You tend to ride the horse's front end and not the back. This is evident at 4:13 when you start to jump, but the horse's back end isn't ready to do so. You don't have leg on and the horse is not pushing from behind. The front end has to stutter step and get in close to the fence to allow the hind end to catch up. This means you are ahead of the horse's motion and up over the fence before he leaves the ground--a serious fault because if the horse stops or stumbles, you are on the ground. Plus you make his job harder because instead of just hanging with him, you now make him push you up out of his way. You can get away with it over these teeny fences at the trot on an honest horse, but my mare would have you on your hiney faster than you can say Bob's your uncle. She taught me to correct this particular fault!
Putting your leg on will also allow you to keep your horse straighter. At about 4:30 or so he's a little wiggle worm--butt sliding left and right. Legs on pushing the horse straight forward and up into a soft, steady hand (and while unsteady, your hand is relatively soft) is the goal. In the meantime at least sending the horse forward and keeping his butt straight will help. He's honest and you do a good job keeping him from taking advantage of the wiggling, but best to nip it in the bud and just ride him straight, straight, straight. You already keep him from popping his shoulder out and tend to keep his nose forward. Now just straighten the bum! This will help your dressage--remember that bending issue? The two are related. Put your legs on, and you can start to control the horse's bum and body position, which will help your straightness, bend, and balance (and your geometry), which in turn will help your left lead. Legs on will help keep you from jumping ahead, keep your hands steady, and keep the horse moving forward and up. Are you sensing a pattern yet? ;)
If your horse has been in some work, jumping a bit, and feeling good, go for the BN. If the horse hasn't been in work, don't jump until you have built up fitness slowly over time.
Good luck and have fun!
I just finished riding, and I actually did a few of those changes, even though I didn't know that they were what I was lacking XD
I really worked on going down the diagonals today, and having him stretch downward and relax. I noticed he does the wiggly worm thing when I try to go in a straight line, so I squeezed my legs on him, and he straightened out quite a bit! (nowhere near perfect, but it's progress). I worked on sitting up straight, and further back so I wasn't leaning so forward. I added in my hips moving with his movement, and my hands too, and it felt SO nice. As soon as I did this at the "free walk" (or what I call it is at the time) he stretched down, and just relaxed. I am FINALLY unlocking these problems I was having.
I am definitely going to try the alternate posting thing, even though it sounds killer hard, since my brain can't even wrap around it. HA HA. I have tried doing only one change of diagonal while standing, and it failed miserably. I'm sure it will definitely help. Also, my chair seat, I noticed I start to have it. My trainer never really mentioned it, but I think I should've known since I had shown equitation for 3 years previous. I am working on my eq. Now, and starting to get my original strength back. Also with me getting ahead of my horse while jumping, just since you noticed it in that video, I will be posting a video from me jumping 3'3-3'6 in a show where it got in the way TREMENDOUSLY. I feel like a complete idiot for not noticing it while jumping only 2ft. It definitely became noticeable a month later in the other show.. ha.. ha..
I will be showing BN in 2 weeks. I just decided not to rush it, even though I would love to begin to go up levels, but I realized I am definitely not ready. About a week and a half ago I was jumping my horse over around a 2'3 jump a few times a ride, so I think he will be able to handle the jumping. I'm dragging a jump out into the arena or field to practice on. (including the approaches so I can stop wiggle worming up to them).
And again, thank you SO much for such a response. I feel like the clouds just parted and revealed my problems so much sooner than I probably would've noticed on my own. I will probably have a video up after the show, and I will link it in here. I really hope there will be improvements. If not for your help, I would probably be struggling even more so than I was before. Thank you.