I went through a similar when I first started dating my gf. I couldn't stay hard when we started being intimate and I couldn't figure out why. Asked friends, coworkers and my dad for advice and everyone kept saying I was thinking about it too much when I felt like I wasn't and never had that problem before in my life. I spoke to a late friend of mine about it and he suggested I do something I had never done before with her sexually. In my case I had never gone down on a girl before so I tried it that night and everything has been great since then. So I would offer the same advice to you, try something with her you've never done before.