So this happened.

I've been seeing this girl for a little while now and we hung out a few nights ago. We started messing around for the first time and she started giving me a hand job. Then I encountered a problem. I couldn't get hard, only semi. Eventually her arm got tired and we went back to kissing. She was cool about it because she knew I was nervous (which I was) but that's not my problem. I've watched/masturbated a lot of porn and now I can barley masturbate without visual stimulation. I rarely watch it now but is there anything else I can do that will prevent this in the future? I know that a blowjob would do the trick but that's not where I have a problem. Any helpful advice?