Instead of constantly pulling in hopes of slowing down do TONS of half halts. You risk the horse speeding up if you keep trying to hold them back as they start fighting with you. You can also do lots of crest release excercises or using a driving reign so that your elbows get softer and follow better. For sitting back, try some no stirrup work, if your leg is in the right spot then the most comfortable position for you in the saddle will be in the middle, once you get a definite feel for where that is then that should help with the leaning. Hope this helps! Let me know if you have any more questions!