Oh Alyssa, this is such a hard one.
The way I see it, you have a couple of choices.
A) Let your difficult child mother back in to your life, but identify a clear boundary in your head which you won't let her cross. Keep topics neutral, don't let her get too close, etc.
OR
B) Put all the cards on the table. Tell your mother exactly what's bothering you, and clearly state the terms on which you want to have a relationship with her.
The risk with option B is that if she's not willing to hear you out, or deal with you on your terms, then you may end up with her out of your life again. Not sure if you're comfortable with that option or not.
I chose option B with both my difficult child parents several years ago. I sat both of them down and said that if we're going to continue to have a relationship as adults, we needed to clear the air about a number of things. difficult child father was livid and tried to take revenge a number of ways, including trying to convince husband to have me committed to a psychiatric hospital. I haven't spoken to them in years and years. Honestly, though, it was a relief to have them put their feelings out there for all to see once and for all. My life is much better without the lies and manipulation, and I don't have to tiptoe around them anymore.
Sorry for the rambling. I hope this helps you with your decision. Whatever you decide, do what's best for YOU.
Trinity