I don't recall you saying but, did she raise you as a single mother?
It was split custody between her and my dad, but she slowly started lessening the time I had with him until it went from a direct 50/50 split in time to only seeing him every other weekend.
I did manage to get away for a bit and stay at my dad's when I worked, but I can't get to college from my dad's, so I can't go stay with him right now. Plus they don't really have space for me anyways. My mom would ask me to quit work every once in a while just so I would go back to living with her instead.
I guess she worries too much about me. Though I disagree with her ways of dealing with it.
And she probably worries about being abandoned, eventually. So maybe she over forges the links to her children. My mom's strategy was accommodation, to a point, and eventually making threats I knew she wouldn't make good on. Conversely, I only saw my dad about 3 months out of the year, all at once, and at Christmas. And he, as he put it, "tried to compress a year's worth of parenting into 3 months" with understandably fucked up results.
You can't ever erase the doubts of someone who is committed to having them. But reassuring, instead of fighting your mom, might honestly loosen things up. If you really care about your mom, that'll come through when you go "hey, don't worry. I'll do ok, because you've helped me." That kind of thing. If that seems in contradiction to the advice I gave above, it's not. You've got to attack her fears from both sides of the spectrum, like making her feel she's been doing some good by you, but also showing her how she's running the risk of hurting you too. My mom was always pretty strong for her kids, but it didn't take much to crack the outer shell and show how stressed she was, and how vulnerable she felt. Being nice and understanding is usually what did it.
It's weird as you get older how you start finding ways to check the people your parents are, compensating in some ways. I didn't do it very well at all until I hit my 30s. My relationship with my dad at least was a post-teenage train wreck, until we had it out in a big moment of unleashing 20 years of tension between us. I wouldn't recommend going that (relative) route with your mom.