A couple of times unfortunately.

One time because the I was dating was a crazy person and talking about our relationship issues or trying to break up had, up to that point, been unproductive. An opportunity presented itself sometime later and I took it. I told the girl I was dating the next day what had happened and tried to end it. Her exact words were "That's ok, people make mistakes, we can get through this." To which I responded "It wasn't a mistake." She's married now, I hope she's well.

The next time was because I used to have this wholly unhealthy tendency to let relationships drag on long past when they should have ended (so really, the same problem as the first time, except less crazy). I had completely checked out of the relationship emotionally. I cared about the girl a lot as a person but I had stopped loving her. I was probably insecure about being single and definitely did not handle the situation well at all. I transitioned smoothly from one girlfriend to the next.

In hindsight, I learned a lot about me and about relationships and how I am from these situations. I try to deal with relationships in a more straightforward and honest way now. I've certainly grown from it. I regret hurting people, I carried a lot of remorse about that for a long time, but I'm a better version of myself now. I'd never do it again. I've had plenty of chances over the years but I actively remove myself from situations that could lead to inappropriate behavior now. Regardless of how I feel in a relationship, no one deserves that sort of betrayal and I can't stand the guilt.

Was it worth it? Eh. In the moment it was fun. Immediately after was not. Long term growth as a person was uncomfortable at times but overall, I like the person that I am now more than I liked any previous version of me.