SUBREDDIT: r/relationships
TITLE: I [27 M] am terrified to start dating because of my terrible sexual performance
POST: *First off, apologies if this is the wrong place for this - if so please just let me know. It does involve sexual details but the focus is on how it's crippling my relationship status*

So I just turned 27, and have been single for 4 years now. I've had a couple (semi-serious) relationships before, but my sexual performance has been terrible in both. I'm decent at foreplay but have issues "keeping it up" and lasting longer than 30 seconds, and I've never gotten better over the course of these previous relationships.

I was lucky enough in the first relationship in that my gf was patient, and stayed with me despite all this. In my second, it was a serious strain, and she eventually left me for my coworker.

It was mentally debilitating then, but now that I'm older it's even more crippling. I'm not too worried about attracting girls, but my confidence is shot because of this and I don't know how to approach a future relationship with this being the reality.

I realize this is open-ended and vague, and would greatly appreciate any advice.
TL;DR: 
Can't perform or last long in bed despite two relationships, and am too afraid to even date again as a result.