Ok, I was on your position three and a half years ago. I was determined not to slip into size 20, but all my size 18 stuff was loose fitting and elasticated. I was definitely 
I joined SW. Did it online once, several years ago coz I 'don't do groups' and lost some for a short period when I was bridesmaid to the world (i.e. 3 times in a year and a bit) but then it went back on when I stopped thinking about it.
I tried doing it on my own for a bit but with no real motivation so I thought, what the hell, and went to a group (when the 16 became 18) and lost half a stone before the healthy lifestyle caught up with me and I got pg with our first.
Then I found out that BFing doesn't help you to lose weight, well, not if you're scoffing cake all the time
I was basically living in my maternity gear and not looking close to shifting into my normal clothes for some time when I went back to the group. Lost a stone and a half and got pg again!
So this last time I have promised myself will be the last time. I am in a group I love, with a fabulous leader. That makes all the difference. The last one was ok but there was no real 'all in it together' group spirit. We all joke and laugh about mistakes made and how life isn't quite as perfect as we would like it to be and we're all really happy for each other's successes. It also makes a big difference that there are lots of target members who keep coming back. Everyone agrees that it's keeping up with the group that keeps people at target.
Anyway, size 18(+) then but now, three and a half stone later, I'm in a 12/14. I've got 6lbs to go and will lose that /14 by then. I'm going to do it for Christmas. I'm going to do it for myself. I'm going to do it for all those new members who come to group looking miserable and obviously thinking it won't happen for them. I haven't done it as quickly as many but I've done it. And anyway, it's not a race (and I'm definitely the tortoise in this one if it is! Remember, the tortoise won...)
Don't despair. That's the thing. Don't have a bad day and think, 'fuck it, why bother.' Have a bad day and then wake up the next morning thinking, 'ok, that was that but this is a new day.' Don't compare yourself to anybody else. Compare yourself to who you'd like to be. Remember the days when you could walk up the stairs without getting puffed and think, 'I could do that again.' Remember the days when you looked in the mirror and said to yourself, 'yep, that's fine, you'll do,' and you could get on with your day without another thought on the matter. Remember the days when you didn't have to worry about what other people thought and now, stop thinking about what other people think!
You can do it. Only you can do it. It can be done.