No expert but here's a couple of things I see right off the bat. :) Firstly, welcome back to riding! I'm sure you just need some time to find your sea legs, so to speak.
1. It looks to me like your hollowing your back and throwing your chest forward in the rise of the post. I would keep a neutral back, think of bringing your belly button towards your spine and allow the horse's hind leg to push you out of the saddle while you maintain that nice line in your spine.
2. Your lower leg is slipping too far back here. You want your ankle under your hip. That can be for many reasons, I can't really tell from the pic. You look like you have all knee in the saddle, no lower leg. You want a pretty even (not grippy!) contact with your whole leg on the horse, thigh through lower leg. You may need to do some off the horse exercises to open your hips up. I do yoga stretches. If you are tighter up top, you are less able to open around the horse at the bottom of the leg. I would not worry so much about keeping your heels down, I would more work on keeping an open hip and making sure I was on a three point contact with the saddle, that is both seat bones, and pubic bone when in full seat. I would also spend some time in two point every day, especially at trot. You can't fork seat very well if you have too keep your weight in your heels! ;)
3. You are breaking at the wrist (also my favorite riding sin!) Try to keep the thumbs up more as if you were holding ice cream cones.
4. Some might say you are locking your elbow from the second pic but I think you are just following your horse's mouth and if it is not a highly trained horse and you are not a highly trained rider, some following elbow is preferrable to a rider who cranks her horse's face down and locks her arm. Do try to keep elbows relaxed and at your sides as often as you can.
Hope any of that helps. :)
~And He created the horse and said to it, "I have made thee without equal."~