Yes, your saddle has slid way to far back. Either your girth was too loose or you need a breastplate to keep it in place. I would worry that this may show that your saddle does not fit your horse.
Now, to you....
You crest release is in need of work. You are pressing your hands onto the top of the horses neck and using it to prop up your upper body. That allows what is, essentially, the laying down on the horse's neck.
As a result of leaning on your horse's neck, you do not need to have your leg in the proper position to act as a counterbalance for the weight of your upper body. You are gripping with your knee which takes the lower leg off of the horse's sides. Since you are so low on the neck, your leg isn't stretching enough forward due to the leg muscles being too tight. As a result, your lower leg has slipped way back. This makes you even MORE unstable in you upper body.
What/ how to fix?
You need to practice keeping your lower leg forward. As you go forward into your jumping position, you MUST keep your lower leg at the girth with your
heels down. This forward lower leg and heel down position will help counterbalance your upper body. The further your upper body goes forward, the more crucial this is.
Try riding in your jumping position with your hands off the neck, out like an airplane. Bring your lower leg forward until you can maintain this position without touching the neck. You will feel the back of your legs burn with the stretch, until you improve your flexibility. Again remember that the heels down are crucial to your balance. Try the position, then put your heels
up and feel the difference!!
When you try this while walking/trotting/cantering, whenever you do a crest release, keep your hands on the side of the neck, not the top. This will give you some support, but not enough to totally lean on. It will also help you maintain a better contact with your horse's mouth without risking a jab.
Let us know how this works for you.
For safety sake....address the saddle slipping issue NOW.