English and Western are like playing the violin and the fiddle. Both are beautiful and both require talent. I have and still would like to ride both though my old trainer sold most of her horses except her eventing prospects both OTTB
OP I applaud you for your DIY kind of mind but please don't feel offended by the suggestions of finding a trainer to help. Most of us did grow up learning from those more experienced than us and you can guess where the more experienced ones learned it from. Not everyone does it this way but I don't think it is anyone's place to judge how one gains experience. Experience is experience. Successful DIY-ers are problem solvers, they figure things out and get things done. I consider myself a lazy bum. I research everything that interests me and I asked questions. Without the "old-timers" I'd have never figured out the cool trick that is removing a rope halter from underneath a bridle ;D. Besides you didn't learn everything by yourself because the idea to ride came from some where. Also the saddle you ride in no matter how old has been developed over time since horses began to be used for riding so show some respect to the many generations of people who passed on their knowledge to get us where we are now:P
The point is that OP, learning is not a matter of pride. Getting some help from someone who has been there done that is not going to hurt anything except maybe save you a pretty penny(like helping you find the right saddle) and save you from the awful pinching that comes with riding in jeans in an english saddle(or am I alone in that? lol.) There are these magical devices called Half-chaps that'll save you from plenty-a-bruise.
Also lets stop all the hate on heavy saddles! My trainer's husband has a lovely old one that I'd steal in a heart beat If I could lift it. It may not be 100 pounds but I sure as heck couldn't lift it several years ago and probably still cant. Exaggeration it may be but not that much of one given I easily carry those bazillion pound hay bales ;).
Now back to the original topic. Here is my advice.
Given you've decided to try English riding that means there must be something that inspired you to do it. Perhaps someone? Study that someone, Pay attention to the angles of their body. from ear to shoulder to hip to heel, elbow to the horse's mouth. While I associate this with dressage and you might try it given I've known a lot of western riders who made the transition to dressage much easier than to hunter, jumper, etc; it would be useful to find the angles so you can mimic them yourself.
Next... Post, Post, Post! at the trot/jog you stand every time the horse's outside leg comes forward. The idea is that you are taking weight off of the leg that will bear the most weight on a curve. In say an hour lesson you want at least half of it to be in the trot not only because this is the one a horse can travel the longest, fastest, but also because it is uncharted territory.
-If you stand and are on your tippee toes with your knees straight, you are doing it wrong(and your stirrups are likely too long.
-If you are bouncing in the saddle you are doing it wrong.
-If you feel the need to pull the horse's mouth to stay steady, you are doing it wrong.
-If you are pitching forward and back and feel unbalanced, you are doing it wrong.
-It you are squeezing the horse's side to stay centered or to keep from pitching forward, you are doing it wrong.
-If you simply let the upward push of the horse's stride push you out of the saddle, you are doing it wrong.
-If you stand then your butt smacks the saddle, you are doing it wrong.
Their are probably thousands of ways to post incorrectly but thousands more to a good quality posting trot. You should allow the horse's gait to give you a little push out of the saddle but not depend on it(especially if the horse's you are riding are flat movers). Your lower leg should be placed firmly to the horse's side but your knee should stay loose and following, pinching them in will pull your lower leg away. You need to have control, rhythm, and balance to use a posting trot effectively and not to mention how difficult it can be.
If you want to move on to the oh so fascinating two and three-point let me know.
As for transitioning your horses.(this may or may not be true given what discipline OP rides vs. what OP Wants to ride.)
Contact. This may or may not be difficult depending on the horse but in most English disciplines, contact is very important. This doesn't mean clamping your elbows and being stiff as a board but you also shouldn't tighten the reins then let your arms flop. You should have a straight line from your elbow to your wrist to the horse's mouth. You also, as I mentioned above, should have good lower leg contact while maintaining that loose following knee and hip. Why? English is very much about precision and invisible aids. You want it to seem like you just thought about what you wanted and the horse did it. It should be THAT seamless and that is why everything is so close contact. This is why other's mentioned taking lessons on other people's horses because those horse's know their job and will respond how they are supposed to (hopefully) because when you ask them they will do what your asking(good or bad). Albeit English and Western are not so different but they are still a slightly different language and it might be hard to teach your horses how to do it when you don't know how it is supposed to feel. I swear every time I switched from a green horse to a lesson horse I got culture shock.
Western walk=English baby steps
In English you typically want a nice marching walk with a little swing through the back. You want to look like you're going somewhere though if you break into a trot/jog you need to bring them back down. Don't punish them for being confused as to what you want. Just take your time.
Western jog=Lazy English horse trot
Again you want your horse move with a sense of purpose. Obviously a lot of people post but English rider's also(or should) use the sitting trot, though it may be different if your used to nice slow jog, add a little impulsion and all of a sudden its a lot more bouncy then you remember(fell off a horse bareback this way back in the day lol)
Western Goin-Nowhere lope=English ?
The canter like all the other's should be forward and going somewhere. Its quite a bit different from the nice little lope(so comfy!) The biggest difference I can see if it is less about flattening out into a hand gallop as opposed to bringing their head and chest slightly up. Not so forward as up and balanced.
One thing that would be good across the board is adjust-ability. You want to teach your horse to go at the pace you ask and stay there without too much encouragement or nagging. This can be surprisingly difficult given a lot of us, myself included tend to maintain one consistent speed and only change to change gaits. In-gait transitions will help you teach and tell your horse what you want.
and also maintaining a steady tempo and rhythm will get you brownie points :)
Lastly, just have fun with it. If you want to gallop across a field do it, if you AND your horses get frustrated with stupid English go back to your roots and get back your confidence. Maybe an English Lesson then do something fun like a bareback western escapade through the field lol. We all started riding for a reason, don't loose sight of that for frustration.
Disclaimer- I am basing this on Western and English pleasure maybe a little bias towards dressage principles. I am not a professional, and if you disagree that's fine. I don't know everything. I'm just giving my bazillion cents.
I wish you the best and I hope to hear about your progress.