I think you need a new saddle too. I know it's hard - I've bought about 8 saddles since buying my first horse almost two years ago. I still have 5, lol. You can pick them up cheap sometimes, if you know what you're looking for. I buy synthetic Wintecs because they're cheap, they fit my horses, and my daughter and I can achieve a half-decent position on them. I know some people will disagree - if you have tons of money, buy yourself a high-end saddle, but if not, the Wintecs are ok. This one is definitely too small for you. There are some useful guides out there for figuring out what size saddle you need, and Wintec has gullet measuring devices you can use on your horse (even if you don't buy a Wintec, they are useful to give you an idea of how wide you need the saddle). You can also do a wither tracing, photograph it, and email it into a saddlery that will tell you what size you need for your horse.
Your position is indeed way off, and this is likely caused by the saddle. Your shoulder, hip and ankle should line up so if the horse is removed out from under you, you would fall on your feet. This takes some practice, even with a good saddle.
It also looks like your horse may be somewhat difficult to fit, but we'd have to see conformation photos on a level plane. Looks like it has a very underdeveloped hind end to me, but maybe it's just the angle.