Your thigh strength will give you that 'lift' from your knees that you need to raise your bum out of the saddle.
Try to think that you need to be sitting 'into' your horse with the rest of your body to be strong and secure - that's where the 'melting down' idea comes in
The 'standing in your stirrups' exercise is a different thing to 2 point (or at least it is in my head' - it will help your balance, strengthen your legs and also put your legs in a good position so the stirrup leathers are vertical and not going back too far or forward too far. If you scroll down a short way in this link it shows 2 pics of someone called Joe demonstrating standing in the stirrups - Figs. 3a & 3b. You need to aim at staying more upright in this excercise
Secure Seat (SM): A Safe and Systematic Approach to Teaching Riding