Okay.. so sounds like you are weak in the hip and upper thigh areas, if your boot drags across your horses back when trying to dismount. You said you can't lean forward enough, and feel unbalanced. That's your"core " muscles that need strengthening,
When you stand up in your stirrups, how far are you out of your horses saddle? I found that once I raised my stirrups, so I could really stand in my saddle- and clear the cantle by 4-6 inches, everything else improved. My legs hurt, as did the rest of me, muscles working harder to keep me in the saddle, for about 2 weeks(riding daily)
Then I added some exercises on a big exercise ball- the one I play with my horse with, and it all started to come together
My coach made me ride 2 point more, scince it works all of the same areas, and I needed the practice.
The bonus for me- it strengthened weakened muscles, I became a more confident rider, and I toned up so lost inches, but no pounds lost for me.