That's a very difficult question. I truly feel sympathy for people who feel so much guilt about leaving their significant others behind.
Guess how I see it though is- to get them to break up with you, you will either have to make them start hating you, or I guess- spring it on them. Either of which will be confusing or painful or both- for both of you. I imagine it takes quite some time to get over someone, so even if you ctb when you are not together, it's still going to upset them I imagine. Personally, I think in some ways it's actually 'kinder' to stay with them and to make as many cherished memories as you can (if you can). Plus, to leave them a note explaining as best you can why you did what you felt you had to.
I think it's unfortunate when couples find themselves in this position but I still don't think it's selfish. Sometimes, we just can't go on.
I guess- what I can't quite get my head around is people searching for a relationship if ctb feels fairly likely. I guess I understand it in terms of recovery- I think forming relationships of all kinds must help in that but if ctb is definitely on the cards, that does seem a bit unfair.
I've lost family members at varying ages. My Mum when I was 3, my Nana when I was 10. My Nana's death was incredibly painful at the time because we had had that time to become close and build beautiful memories- there was so much more to miss. That said, if I had the chance to have somehow had my Mum for longer- so that I would have some memories to cling to- I would choose that. I know it's not the same but I guess it's my thinking that it's more preferable to have some good memories rather than conflicted ones.
It's an interesting post and I think it's something people with any kind of ties considers.