First of all happy birthday. I'm sorry your mother has soured a day that should have had you celebrating.
Your situation is something I can relate to all too well. Just a simple phone call, or even a text message or e-mail from her tends to have me brooding and miserable most of the day, so I can understand where you're coming from. In fact phone calls are the worst for me because I have to listen to her voice.
Don't let her guilt you into thinking you owe her your companionship. You've made the decision to move on with your life, and you have the right to do so. You also have the right to protect and defend your boundaries where necessary, and this is what you're doing. It is sometimes hard to forget, but no matter how old you are, at the end of the day, you are the son - your mother is the one you're supposed to turn to for encouragement and emotional needs, and NOT the other way around. She is an adult, and therefore responsible for her own emotional well-being - it is not your responsibility. That's her responsibility, and it is up to her to seek a support network, from friends or family, so that SHE can own up to her role as a parent and be able to take care of you. That is obviously not happening, so justified in every way distancing yourself from her - you have got the right to fight for your own independence, recovery and happiness, there is nothing wrong with that.
I know it is rough getting triggered like this, but know that I'm in your corner.