Sex... doesn't work.

Missionary (normal, me on top of her) position will not work. First and foremost, my penis won't stay in. It may be attributed to the fact that I have a slight downward curve I in. I also can't seem to get the thrusting motion correct. Doesn't feel at all natural and is very laborious when I try to thrust. Every other position works fine. But I sorta want this to work. IIN/Suggestions?