I would actually disagree with other critiquers - your seat is actually in a better place than most. Leaning forward is a bad habit to get into, and right now I would say the position of your seat and leg is the best thing in your riding.
Overall I see a lack of positive tension in both horse and rider. The picture is one of a horse who bumbles along passively and a rider working on getting her balance through use of the reins.
What I think is going to benefit you the most is shortening your reins a good 8" each, and then pressing your hands forwards and towards the withers. On a horse who is so behind the leg and tends towards on the forehand, the hand needs to be extremely passive, pressed forward and never used in a backwards fashion.
From there, really ride the horse forward into a working trot. If you are sitting, your abs should be burning. If you are rising it should be at a far quicker pace than what you are accustomed to. The trot you are riding now resembles more of a jog, and with the length of rein, how much you fuss with your hands and how little leg you are using, the horse is falling further and further behind you. You can see, every time the horse tosses her head, your elbows come out and back and then you are pulling. That is riding front to back. Back to front riding dictates that is the point at which the hands soften forward and you kick the horse forward.
Overall I think your seat is quite good, and now it is learning how to ride correctly and getting the horse going forward in a more positive way.