what is it that they say...
the brain is the largest sex organ?
there have been reports, i personally can't verify them at all, that there are some women, though rare that can have orgasms from stimulation to the nipples.
how does that apply to you and your girlfriend? i don't know.
however
i know that i often have the same problem she does and i know for a fact that it is totally a mental thing for me. um its not that i don't love sex and think about it constantly but its really difficult to let go of rational thought for me.
practice though and relax. sometimes when my boyfriend is going down on me i start to worry that i'm taking too long or hes getting bored. and then forget it that makes it even worse.

i'm working on developing a whole new attitude about it. my new thought is that i deserve the attention and i better damn well get it. that has really helped
but like i said. most sex problems are in the head.