Venous leak is a possibility - however, here's another Q for you: Do you have any trouble getting erections from self-stimulation? If so, that would indicate this is more of a psychological prblm than physical. You have said yourself that you get nervous & wonder if it's going to 'happen' - that sounds exactly like so-called 'performance anxiety', which is when a guy starts thinking too much about whether he's going to 'perform' (terrible term!). It's also occasionally possible for guys to have prblms getting had even during self-stimulation due to so much anxiety about performance.
Another Q - do you still wake up with erections, either in the morning or late at night? If so, that's another sign your prblm is more psychological than physical.
I would advise seeing your reg Dr. first - I know it's embarrassing to discuss this issue, but remember - it's extremely common! Just look at the number of ads for Viagra, Cialis & even 'natural' treatments for ED! Your Dr. won't be at all surprised by your Q, and in fact, for all you know, if your Dr. is a male (especially a bit older), he may suffer from this issue himself! ALso, because it could also be an early indicator of blood they need to know if there is in fact a physical - pressure, circulation & cardiac issues. If you reg Dr. can't solve the issue for you, get a reference to a good Urologist who deals with ED issues.
I wish you luck with this & please come back here & let us know how you're doing & what was found...
Your welcome my friend, anytime. If you have specific questions, I would be happy to provide you with an email address for a more private conversation.
It is very likely that it is a venous leak. To start with, you MUST tell your partner, and not just before you go to try, that will just seem like an excuse. If you don't tell her, you will have psychogenic ED on top of whatever else is going on and that will be really hard to fix because if they treat for both and it doesn't work, you wont know which didn't work or if both didn't. Explain it to her... women are really understanding when you don't blindside them.