I think you need to realize your mother is in an abusive relationship with drugs. Whether or not she is using, she is always going to be an addict. Until she wants to get out of the cycle, she is not going to be your mother. She is not going to show up to events and if she does, she might ruin them anyway.

I don't think you need to tell her you are ending the relationship. I don't even think you need to end anything. She has already ended it. You would just be wasting breathe on her.

I think you need to stop having expectations she cannot meet. Be civil, but treat her like a roommate your grandma happens to live with. Don't invite her to events, don't invite her into your life more than you would normally. I think this will allow for four things to happen.

1. If your mother goes back onto drugs, you won't feel like your words drove her there.
2. You won't be bringing drama into your life or being blamed for her addiction.
3. It allows you to approach her in the future if she has shown herself being sober for 6 months plus.
4. You don't put your grandma into an awkward position trying to choose sides.

What worked with addicts in my life was making them start out as a stranger. They don't get invites to personal things, they don't get to know the personal details of my life from my own mouth. What they get is minimal, polite, reserved contact. I don't loan money, I don't give rides, and I wait for them to get sober or get lost.

It also doesn't make their problems my problems. I know this is your mother and it's really hard. But she is showing you what she wants out of the relationship: nothing. She doesn't come to your events, she doesn't support you, she didn't even raise you. So back off and let her have what she is showing she wants. Her actions say she wants no responsibility.

So don't feel you have to do anything. Treat her like a stranger.