Were you two actually dating? having re-red the OP it sounds like you were more of a stalker / think boyfriend. i mean if that behaviour was going on with your GF for more than 1 day you would be proper suss. Sounds like it was going on for a while, i.e. not getting allowed round hers or her answering phone/returning texts.
You sure this other guy isn't her long term boyfriend and you were the 3rd party all along?
It was a full blown sexually active relationship but we do not live together. It was not some school yard crush. Perhaps my OP wasn't as succinct as it could have been, but I was emotional at the time.
Also many have said there is always two sides but the fact is she never even gave her side and just cheated. What hurt the most was the lies and deceit. We all know you can't change the way someone feels but I think people at least deserve some respect.
Anyway, I'm well rid and all my friends, family, and brothers on OCUK have been very supportive.