You're 18 and unless your issues directly affect the relationship, your living arrangements or anything, there is no reason she needs to know. It would be ideal to be able to talk to our parents about everything but the fact is that's rarely the case.

I would honestly ask yourself why you would be motivated to tell her about it. If it's for support, then there is no reason that because she's your mother she would offer the best support. Everything she's done and said seems to point to the opposite and someone that might add to your problems by not being supportive but judgmental.

If it's to be closer to your mother then I would say just because she doesn't know your MI, does not mean that she can't be close to you. there are other ways to gain this closeness with your mother.

In all relationships we have to pick and choose the ones we share various subjects with, do different things with and who can be trusted with what. A parent isn't necessarily the best one to share mental health issues with and in fact many times they are a bad choice for this.