It sounds a little like performance anxiety. Other than suggest you speak to your doctor or try some psychotherapy, or wait til this forum expert responds, I would likely recommend you search the mental health international expert forum archives or post on the men's health community forum or erection concerns expert forum or even the anxiety community forum. It's lame advice but hopefully you can find some additional advice through those forums. You may also just like to try Googling it.
Maybe you're just not ready for an intimate relationship at this point? Sometimes there are psychological reasons behind the problem. The answer could lie with them.
Sounds mental vs physical but you could always get that checked out too.