Chin up Mart mate, I know it can be such a vicious circle and sometimes we beat ourselves up worse than anyone else ever could because of our own deep-rooted insecurities.
I joined a new work in June of last year too and for AGES tried my best to fit in, I was a complete outsider who knew no one else prior to joining so I wanted to hard to feel like I was accepted as part of the group. It never happened and for a long time I was down about it. It wasn't just conversations I wasn't included in, the lot in my work spend their dinner hours together and eat in a completely separate room with the door shut. As if to say "Don't come in here and disturb us, you're not welcome." Or the Christmas night out where they all booked an apartment to sit in for the night and near enough everyone was invited except me and a couple of older blokes with wives. It made me feel like such an loser, I was so depressed about it for a long time and I posted in here about a fair but. Not one to this day has ever been outright horrible to my face because they're cowards, just subtle behaviours that continue to make it clear to me they don't have any interest in building any sort of friendship or even a friendly working environment.
I've managed to get over it largely because the more I described the situation to people (whether it be in posts on here or close friends and family) the more I began to realise it was THEM in the wrong, not me. Literally everyone I've told about it has said "They sound like a right gang of idiots, just steer clear of them". As time has gone I've began to agree with that, they are a gang of p***ks because most people wouldn't treat a new person they way they've treated me, it's outright rudeness.
Your situation sounds very similar mate, these people haven't given you the time of day and it's confused you because you know if the roles were reversed you would be a lot nicer to the new person because that's the type of person you are. My advice would be to just live your life as best you can outside of the place to keep your mood up. Work to live, don't live to work. Enjoy the good times with your girlfriend and your mates as best you can because that's what life is really about, making boss memories for ourselves. We choose to spend our lives with the people we love and like but unfortunately most of us have to put up with having a few divvys on the fringes as well, we just can't let them decide our moods. I love going to or watching a match whenever my work troubles get me down, for me it's the perfect tonic for my fragile mind. Few weeks ago I asked another girl out, one of the idiot crew, she rejected me and I found out afterwards she and the rest of them were laughing about me behind my back. I went straight from work to Goodison to watch us play City and by the end of the game my troubles were gone, the ecstasy of us winning wiped away the crappy feelings.
You've got an awful lot to feel positive about mate, just sometimes we can't see the woods for the trees and need someone to help us see it. Stay in touch on here or on twitter, they'll always be someone here to vent too.