wouldn't it be nice if we stopped being our parents' children once we reach adulthood? When talking w/your mother, be sure to speak to her w/love & respect. "Honor thy parent's..." & all that. Use I statements of need, feel, believe, think, want. For example: "Mom, I know you're acting out of your concern for me & I appreciate that, but I need you to ..." If she wants to discuss it -- & given that she's your mother & you are her child regardless of how old you are, is highly likely -- then stick to "I statements". If you wish to explain to her why you want her to mind her own business, focus on how you feel & what you think of her interference. Such as: "Mom, I need you to understand that this is my life & I have to live it as I see fit. When you interfere, I feel..." Feelings are 1 word adjectives -- embarrassed, humiliated, frustrated, angry, happy, & so on. One of the best parts of sticking to "I" statements is that she cannot argue the point because as much as she may want, she isn't you.
From personal experience, it's best to stay away from "You" statements. They tend to sound critical & disrespectful, even if that isn't your intention. So, figure out what you want to say to her then practice it in front of the mirror. If your friend is willing to get involved from the perspective of helping you, practice it w/her.
Seems as if she hasn't allowed you to become an independent adult, separate from her. That's a whole can of worms that needs to be addressed @ length. Therapy is a good format for that. If she continues on this track, regardless of your expressed needs & wishes that she stop, then she isn't behaving as if she has any respect for you. Another good one for the therapy MO. Now for the hard part, the therapy would be for you! Learning how to cut unbreakable apron strings is a tough road, but sometimes necessary if our parents insist on treating you as if you are incompetent. If she does believe that, then perhaps she worries that she did a poor job raising you. That can be eased w/reassurances like, "Mom, whatever mistakes you may have made, you did the very best you could & that's all anyone can ask." & so on.
As for the therapy for you, that's because you can only change yourself, not her. You can only be responsible for your own behavior, thoughts, feelings, beliefs. If you are set on changing her, you're going to wind up w/a concussion beating your head against that brick wall.
Hope this helps. GOOD LUCK!