Hi.
Everyone enjoys different ways of having sex maybe you just need to try a different position?
The more you worry about it the less you enjoy it as your stressed and can't let yourself go. Lots of girls have this problem of not really knowing what they want and if your bf has not had lots of experience before then he won't know either.
If you enjoy the forplay try to get him to go down on you untill your really enjoying it and then stop.
You might want sex more then and maybe enjoy it?
Asfar as past relationships go the only way that would effect you is due to you thinking about it during or whatever.
Just focus on how much you love him during since you seem to and forget about everything else and just enjoy it as much as u can don't try to hard but if it still is not doing it for you talk to your bf in a nice way like i love it when u do this to me but sex needs a bit of work on together.
Hope it goes well