Obviously, you are insecure about it, that what makes you go in to defence. If you have the mindset of it doesn't really matter, you wouldn't.
Now, what your problem is, it's all in your head. You know sex is coming, and in ur head you start to worry, 'ooh no, i hope i don't get soft this time'. While you are having sex, in ur head it goes the same 'ok, i'm good now, but it will go down'. From the point you get these thoughts, it will go down!
When having sex, close ur eyes, and think 'this is the best feeling in earth', 'i'm having sex with the girl i love, she's so fucking hot'. Also what's a good one, 'How the fuck can't i get a boner from this, feel it!' I had same problem a while back, when changing positions. But these toughts help for me.
It's also a vicious circle upwards. When you do it good one time. You get the affirmation. You see i can do it. What the fuck was the problem!?

Be glad ur girl supports you in it, so you can try things out. But when you can get hard on ur own, there is no problem. Close ur eyes, imagine all the things that turn you on about ur gf, and just
enjoy the feelings of sex.
Also, do you masturbate alot?
Hope this helped.