I haven't regretted it at all.
Obviously I regret that it was necessary, that I couldn't hang on in there for ds2's sake, but other than that, I have no regrets.
But then not only were STBXH and I not wholly compatible to start with, I'm not 100% sure that I should live with any man. I like being on my own. That's not to say I don't want to be with someone, just that I'm not sure that I'm any good at being with them day in, day out, IYSWIM.
However. STBXH and I, and his parents, have really pulled together to make sure it's a painless for ds2 (8) as possible. STBXH has moved in with his parents, who have made over a bedroom for ds2 (he has been used to going there all his life anyway), until he finds somewhere suitable to buy. Ds2 and I are still in the family home, where I will stay until ds2 leaves university (or I earn enough to buy somewhere else). Contact has been agreed between us, and is totally flexible both ways. Family occasions are still shared, and I suspect this will always be the case, even if either I or STBXH get together with someone else.
So it can be done, if everyone can work together.
I don't get lonely. I have hobbies, good friends, and siblings on the end of the phone/email if I need to chat. It was 100% the right decision, for me. But it was a decision I arrived at after trying everything. The crunch came when I would wake up dreading the thought of being with him for the rest of my life. Not fair on me, and definitely not fair on him.