Yes, I have and I regret it. My situation was not like the above (cheating, drugs etc.). It was because I thought the grass was greener elsewhere and it wasnt!
It was my first big relationship after divorce (which was 5 years back). Fell in love with him quick and it was great. It showed me what a true relationship felt like - felt so loved back, everything he did was unconditional, no nasty words, no drama. Everything was there, our families got on, friends did. Was he the best looking, no, and he was bald which was not my type but I did love him and would trust him with anything.
But, after 18mths or so I started to get bored. Admittedly we both had high powered jobs so tired in evenings, mainly in front of the TV and we didnt make any effort re; date nights etc. Some of my friends were saying how exciting their dating lives were, seeing guys, having real fun and excitement... so I decided that I wanted that excitement too.
It was tough letting go, he really didnt see it coming and was very upset. Sure, initial excitement was there, and a couple of dates with very attractive guys (one of which I took to bed the first night). But then it hit - it was great for short term fun but none of the guys had his presence, of making me feel warm and safe (which I did from pretty much day 1). One looked great but lived in a human pigsty. Another had limited conversation and was dull.
After a few months I realised what a mistake I had made. I had security, love, he would do anything for me and I had chased short term fun. I did try to get back in touch but he was not interested - why? Because I just left, didn't give him/us a chance to discuss and then make some changes to see if it would work. By all accounts he has met someone and seems happy enough but we had something special which I think could have lasted well into old age!
So it all depends in why you want to end it while in love - DV, Abuse etc... of course leave now! If it's because of money / age gap / long term plans / looks then give it a go as you have a lot to lose. It also depends on how old you are? 20s, plenty of time to find someone and lots of options. 40s-60s, the good ones are in the main taken. And while making the Karma Sutra volume 2 or having a George Clooney on your arm may seem like a good idea, that will not last as you get older. Also if you are around the menopause age (I was) that could also be a factor, I think it was for me. Now I am through that I would never have done that if that situation arises today.
More info on your circumstances would help get better advice