I understand what you feel. It is hard to resist another. It's like others said not worth risking your relationship over though.
Also, think of it this way (I would anyhow..) you won't enjoy carrying it as much if your fiancee won't be able to look at it with pride (as in: 'look at my pretty girl with her pretty bag'). This way, the bag will probably loose it shine pretty soon and that would be such a waste!
Focus on the other exciting things that are going to happen in your life right now.
Besides, chanel will always be there, maybe next year with an even more beautiful bag in it's collection you can't resist, start daydreaming about that! Good luck and let us know if you're holding on!
Thanks Roxana!!! Yes I have decided that I will probably wait till next year to get my next Chanel bag (unless of course I strike the lottery this year!). Fingers crossed that my SA doesn't manage to find a luxury bowler for me, otherwise I would find it very very hard to turn it down!