Should you propose to your ex-boyfriend?

Without knowing any of the specifics of your particular case ...

In general, this seems like an exceptionally bad idea. She's an EX girlfriend for some reason; proposing to her is basically saying "let's ignore all the problems we had that made us break up in the first place, and bind ourselves in a legal contract that's difficult and expensive to get out of and will socially stigmatize us when the relationship inevitably fails again, because how could that possibly go wrong?"