Issues with some sex positions

Hello all I'm new to this, hoping for some tips and advice.

I'm 21 male. only been having sex for past few months with my girlfriend. So I am inexperienced. I really seem to have problems trying to have sex when on top, missionary etc. I struggle to get my penis to enter the vagina properly, and whenever it is in and I try and push it in I find it hard and then lose my erection. I am 5 inches so I know this is considered a bit small so I'm not sure if my size is causing this problem but I don't have any issues when my girl is on top riding me, she has great stamina and can ride me for 20 mins or so.

I don't want it to seem as if she is doing all the work, really want to improve and worried she could lose interest! now with doggy position again I have problems with this a bit too. Only positive is at least sometimes I can **** in this position for few minutes but still that isn't good enough. I can't do it whilst kneeling on my knees as it is so uncomfortable so I have to do it in the standing up position, I do though sometimes have the problems with sometimes can't get my penis in properly and then losing erection straight away. I also always wear a condom when having sex just to clarify.

Has anyone else ever experienced any of these issues before? does anyone have any advise they can possibly give?

Thanks