I agree with some of the posts here. Especially what LimeSlice said about your age difference. It might not seem like a big deal now but if a 21-22 year old came sniffing around my 15 year old daughter I would call the police. Either that or set my DP onto him!!! lol Especially as it is actually statutory rape until you are 18. Similarly, if he has ever hurt you physically (whether it be a push, a shove, slap, hit or punch, whatever) I would probably never be able to forgive him if I was her. Mothers want what's best for their children. It's hard to forget, let alone forgive things that have gone in the past when it comes to our babies.
Hmmmm, yeah that too. My post was made on the assumption that you ARE in a respectful, loving relationship with someone who treats you right. If Jazz was with someone who had intentionally hurt her, physically or emotionally, I certainly could never be anything more than mildly civil to them, at least not for a LOOOOOOOOOONG while, and not until I was quite sure that the person in question had actually made steps to change and work towards a respectful, safe relationship. If you are venting to her about him being lazy, inconsiderate etc, she probably thinks he hasn't changed and you are still being hurt.

The age thing - is hard. Once you hit your 20's it doesn't seem to be a big thing, but I think the difference between 15 and 21 is huge... Still, you are in your 20's now, and even though your mum probably (understandably, I am sure I would too, and question the motives of a 21 year old man trying to date my 15 year old school girl...) felt he was taking advantage of you, she needsto open her eyes and see that you have been together for a while and made a committment of an engagement and a child.