I've been with my girlfriend for a couple months now and we have sex more than most couples, on average of 2-3 times a day every day (1-2 in the morning, 1-2 at night). I don't last very long (only a couple minutes tops each time, which is not very good for me tbh) but the problem I seem to be having lately is sometimes we start off with her on top, then shift so I'm on top either missionary or doggystyle and during the action of shifting to me on top I seem to ALWAYS lose my erection lately. She's very patient with it and will go back down on me or play with it to arouse me again so we can go at it some more and sometimes it comes back, but not often because I've become so self-conscious about it.

Is this a stamina issue? Am I thinking too much? I'm 6'7" 250 lbs and in ok shape, I don't smoke, rarely drink and don't do any drugs. She's 5'4" and 110 and I just can't figure this all out.

Please help as it's quickly becoming a mental issue as well and I'm getting discouraged completely in even trying to roll over on top or doggystyle or some of our other creative positions. As a side issue, when I do manage to maintain an erection on top (which doesn't happen at all lately) I cum very quickly as I'm very excited with the way it feels and the way it turns her on. She REALLY enjoys me on top and doggy-style and rough hard sex and I just want to please her, but I just feel like I keep coming up short and it's frustrating.

Please help =(