Hi, firstly you are NOT a failure, you are NOT pathetic, you are NOT useless whatever happens!!!
Now the fact that you can get an hold morning erections is making me think that it's a bit more psychological than physical. Sure trying to reduce the masturbation might help if you can try it and the cock ring is a great idea but as well........I'm guessing that you've been thinking about this a lot and the stress, anxiety and pretty much expecting it to happen really isn't going to help. I know this may sound next to impossible but maybe try turning your focus on other parts of your life, passions and feeling good just being you and take some of the importance off this a bit.
And perhaps your partners in the past haven't helped, with the last one that's crystal clear but were there any some difficulties in your other "relationships" as well?
Might even be as well that you'd really put way too much pressure on yourself thinking of how you were going to "perform"? Sometimes that can be consciously sometimes less so.
So....for the next relationship, do you think it would help any if you had a deeper attachment, felt a bit more trust, felt "safer" with her. Maybe take things REALLY slowly, not so much worrying as to IF it's going to happen? And you know as a backup that if it doesn't happen the first/next time there's PLENTY of other ways to fulfillment for both of you, and PLENTY of ways to build on!!
Best wishes
Alison