Well.. it seems to me that if you can get hard on your own..then there is some underlying anxiety that happens when you two are together....
Mastrubating so much is normal at your age..it really shouldn't make a difference..... you could try to not do it for a week and see if that works....
Your penis seems to be saying that somethings not right.... is there any other problems that you can think of that could be effecting it... Like parents walking in.. or anxiety that you wont be able to do it..or do it right?