Quoting rpgmomma8404:" It's an idea but I'm just having really conflicting feelings right now. I've been with my boyfriend for ... [snip!] ... back 12. Then low and behold someone has to walk into my life and make me want to get out of this relationship even more. "
If you were unhappy before you met/were interested in this someone else, I wouldn't say you were leaving your current SO for someone else, but that you were leaving him because you have been unhappy for awhile.
Leave your current man if you are unhappy. The amount of time spent with him means nothing. Whether you get with this new guy or not is beside the point.