Looking at your pics, the first problem I see is that your heels need to come WAYYYYYY down. Right now, not only are you not pushing them down, they are actually higher than your toe. That means you have NO
support for your lower leg...no wonder it is hard for you to trot! Your stirrups are your base of support and to use then correctly, you need to let your weight sink down and into your heels. They need to be at least an inch or two below your toe.
The second problem is that your leg is actually a little far back (not a lot, but enough). When your leg is too far back, not only does that mess up your aids to your horse and make it harder for you to encourage good forward propulsion, it also undermines your balance and support, again making it difficult to trot and even to sit properly upright. That's why when a rider jumps and they let their legs slip backward, their upper body falls forward and they can lose their balance. I actually think that if you fixed your heel, your leg will naturally shift forward into a better position.
My thought is that your saddle is on the small side. Your knees are almost coming off the front and your seat is actually on the very back of the saddle. That is forcing you into a somewhat tilted forward position that, along with your heels, is probably contributing to your leg position. One that is an inch or two larger would likely fit you better.
Coming to your upper body, it is tipping a little forward, about in proportion to how far your leg is too far back. So, you fix your heel, your leg will fix and I suspect your torso will follow. So now, voila, you'll find that you are vertical and sitting on your seat bones, your knees will stop pinching and you'll be able to post from your legs, rather than, as I suspect you are doing now, from your knees. That should make the trot much, much easier to post. Isn't it amazing how one little thing, in your case your heels, can throw everything else off?
Finally, roll your shoulders up, back and down. Think about making your shoulder blades touch. They're kind of hunching up and rolling forward in a couple of the pics, although they look pretty good in the top one.
I suspect that when you get to this position, you will find that close contact saddles suddenly are much easier to use. In fact, I would almost suggest you use a close contact without knee rolls, because that will take the crutch that is the knee rolls away and force you to make sure your legs are in the right place.
Hope that helps. I know it's a lot, but basically, just concentrate on getting your heels down, down, down and the rest will likely follow.