I found the hardest thing was not having all those friends on tap, and it is hard making new friends once you have no 'group' to identify with. Its possible that you will make a new set of friends through your girlfriend though. Ones who wont drop you like a hot potato as soon as you decide you dont believe exactly the same things as they do.
Regarding family, its amazing how 'blind' they can choose to be as long as you dont rub their noses right into your new fun sins! My friend has a very strict family (brother in law is an elder, sister is a pioneer) and she was convinced that they would cut her off when she got a relationship with a non JW man (even though she had left JWness years ago). But actually while she was just going out with him they just pretended like it was platonic or something.
Then she decided she wanted to be with him properly but felt that she had to marry him or else the family would cut her off. She agonised over it, because her man didnt want to get married (it would have been his 3rd time). Anyway, eventually she moved him into her house and amazingly the family gave a little frown, stated they couldnt stop overnight at her house (which was impossible anyway as it was as 1bed house) and continued to associate with her!
As regards losing friends who are JW's, well you might but then again you might not. I was one of the JW friends who never actually broke friendship when all my friends were leaving and doing really great fun things like lying in on Saturday morning, and having sex! Eventually after another 16 years I left too and we have all remained friends. On the other hand if they cut you off, sod 'em, they werent really proper friends. And then again, years down the line they might leave and you might get em back. Its taken 20 years but virtually ALL of my JW friends that I had when I was an early teenager have left, and we are all back in contact as non Witnesses.
Good luck with whatever you decide, but if you fade gracefully without bad mouthing the religion it may turn out to be a LOT LESS bad than you think. I just pretended I had depression, everyone felt sorry for me, and I got to leave without too much fuss at all.