Liking my best friend who has a girlfriend.

So, this guy and I are work friends. We've known each other for a year now, and in that year, we have become very close, pretty much best friends. He is not your typical guy, he is very much an introvert, doesn't have many close friends, he has depression and anxiety issues too. (I never judge him, and he doesn't judge me, regardless).
So, since December last year was the time we really spoke every day, for long periods of times. We've spoke obviously previously, but not every day in full conversations and random topics. It takes a lot for him to open up and trust someone. Previously we were having conversations where it would take up to two hours trying to replying to this damn messages! (This was going on for awhile). Then we started chatting with out huge 'novels', which was more enjoyable. I mean even now we speak like we're writing a novel, it just escalates so fast. But anyway, to this day, we 'Snap' everyday, (for those who don't know what snap is, it's an app that you take photos, videos and message, and send it to each other, and the photos or videos only lasts ten seconds. So we literally do that for up to eight hours in the day until work, then again after? Every night we speak, for hours. (He has insomnia, and I don't sleep well either). So we chat a lot in the early hours of the morning or all day on our days off.
He has issues with seeing people, he's made a huge effort to catch up with me before work for a few hours. We will go chill, get some food, laugh, it's so nice. I really cherish those moments.
We speak very in depth about everything. We are very open.. I have literally told him everything about me, besides me having feelings for him.
When I first met him a year ago, there was a sudden attraction, and I was hooked.
He has told me, he takes awhile to develop feelings for someone. With his girlfriend, they were also best friends before they started dating. She told him first that she liked him, and it took him a good six months to develop feelings for her back.
Him and his girlfriend have been dating for three years now.. they have never lived with each other. He is 24, and she is 29. She rents with friends, and he lives with his mother still. But this year they are moving out and renting, he told me she is 'pushing' it more then him, but he feels it's the 'right thing to do', and his mothers lease is up at their house, so he has to move out before then, which is August.
Anyway, everyone at work suspects he has feelings for me, by the way we are around each other, he gets all close and a little touchy, obviously not overly touchy cause that would be weird at work. We talk sexually a lot, we are very open, and speak about whatever comes to mind. We are such dorks together, we make each other laugh constantly. He said the other day, that I'm his only comfort at work, and that our shifts together are his favourite. We speak a lot about valuing each other, and how much we mean to one another. It's taken a lot for him to feel 'close' and 'trusting' to someone so fast with how he is.
I just don't know what to do, I want to tell him about my feelings, but I'm scared. I know he wouldn't disown me or anything, I just don't want all our silly jokes and topics of conversations to change. I mean, we speak everyday. That to me, is huge?! I couldn't possibly lose someone that I'm so close too.
Also, his girlfriend had no idea I existed until recently. How can someone be so close to someone and not tell their girlfriend? Who talks to someone every day and not mention it to their girlfriend? I told him I was a little shocked and almost offended she didn't know who I was.. His excuse was 'he doesn't talk about work to her', she I said, 'I am not work related, I am your friend'. Anyway, we worked it out.
So, this guy, doesn't drive (cause of his anxiety), works in a dead end job like me, (not knowing what to do with life), hates himself, literally self loathes himself, and can't see why someone would ever find him attractive.
There is so much more to write, but I wont. I just need advice...
Do I tell him, or leave it? I don't want a 'What If...', but what if he doesn't feel that way and I make an ass of myself. I get very anxious thinking of telling him... I have been meaning to tell him for months how I feel, but I keep chickening out.. I don't want to be the 'other girl'...


URL: http://able2know.org/topic/324845-1