You said "After we had kissed he told me he had a girlfriend but being drunk and stupid I carried on" ......
..... leading me to ask were you drunk and stupid for the next 8 weeks ?
Sorry, but I agree with MacDoodle ... you knew the position he was in (at least he had been honest with you, if not his fiancee) so what you're feeling now is of your own making. I don't understand why you would have carried on after the 1st drunken fling ..... and refuse to believe that you fell in love with him instantly.
Can't you see what a rat he is ? ..... saying "it's up to you" whether or not you "keep in touch" indeed. What he really means is that even once he's newly married, he wouldn't be averse to a shag on the side every so often but by saying "it's up to you" he absolves himself on any responsibility should you end up getting hurt. Wow - he obviously thinks so much of you ... he only sees you as a piece of meat.
I pity the poor woman he's getting married to and TBH, if you know anything about her, I would be seriously tempted to drop her an anonymous note to try and warn her what she's getting into. She may dismiss it as a hoax but at least you would have tried to save her from almost certainly a very unhappy marriage based on a lie (as you say).
For goodness sake, please don't be tempted to get back in touch with him - or respond to any future communication he may attempt. He isn't your ideal man, I don't think a man like this would be anyone's ideal man .... though I suppose the local brothel may get some business out of him on occasion. If you're finding it hard to get over him, then for god's sake, get angry with him - because you're jolly well entitled to be ..... do you enjoy being used ? Then keep telling yourself that you deserve far far better than this - who wants secondhand goods ? - and who wants to have their children getting to know and getting involved with a scumbag like this. Surely that's not the sort of role model you'd wish them to have ? Keep seeing him and your own relationship with your own children will itself become based on a lie, because, presumably, you wouldn't tell them he'd just got married ?
Believe me there are loads of normal decent men out there. I know (from having been a single mum myself, for 9 years) it's not always easy to meet them but it will happen eventually, one way or another. You do not need to sell yourself and your kids short by allowing a scumbag to cheat (if everyone said no to attached people, they'd either have to get over it, or sort out what was making them unhappy in their own relationship).
Concentrate on doing things for you. Again, not always easy with kids and/or lack of funds, but even that old cliche of pampering yourself with nice scented bath, candles, face pack etc does help, and reinforces your own sense of self worth .... meaning you are less likely to settle for any old shite thrown at you. There are loads of other comparatively cheap or free things you can do for yourself - just sometimes we need reminding to actually do them, as it's all too easy to concentrate on the kids and neglect yourself. The more you can keep yourself occupied, the less you will brood about him ...... whatever interests you have, be it a particular TV show, knitting, or rock climbing, there will be several online forums for example, which you can log onto after the kids have gone to bed (IME, prime brooding time) and chat away to your heart's content about something you enjoy.