I was also the always down to hang/party 'friend' and I had a lot of friends too! You can check my thread history on here and see how this **** gets worse! Make a plan and stick with it.. "no thanks" is all you have to tell your friends and trust me, If you're on the internet looking for advice on this...They likely already know you have a problem and I'm sure some of them do too! Make a plan and work the hell out of it!
Edit: By the way..This is a GREAT site with lots of support and advice! I only wish I'd paid more attention to what I was told when I signed up.