I'm sorry you're experiencing this, OP, it must be very wearing.
I think you need to be a LOT more blunt with your mother - have you ever explained how you feel to her in as much detail as you have above, including things like you feel she doesn't recognise you for any of your accomplishments but only "sees" your weight when she looks at you? I would use your OP as a draft for a letter to your mother:
Mum, you are my rock and support me with everything and have always been there for me. But ever since I started to be overweight you talk about it constantly with me, you ask me what I have had for lunch, what I’m having for my dinner, if I’m sticking to whatever plan it is I’m trying this time. You just see my weight when you see me and I find it quite upsetting. I have spoken to you before and asked you not to talk about my weight and you do back off a bit but then it slowly starts again. I am proud of you for your weight loss but I am not you and I don't want to diet.
I’m currently smashing a degree at uni, my kids are well looked after and happy but I feel like you don't see me for any of these things. You always seem so disappointed in me, like I could be the president of America but it’s still not
good enough because I’m fat!
I love you so much but discussing my weight, diet or anything related is completely off the table. If you mention it I will ask you to stop. If you continue I will hang up the phone or leave.
I hope that we can continue to have the same wonderful relationship that we have always had, but without my weight being this difficult topic between us.
Love, toomanyjobsin1day
At the moment I think you are wanting her to change (which is completely understandable but not going to happen). Having good boundaries with other people means accepting that you can't change who they are or how they behave, you can only change your own behaviour.
If you tell her the above and then follow through on your end (refusing to listen to her talking about your weight by hanging up or leaving, or asking her to leave if she's at your house) you will find that either she does learn to stop bringing it up or at the very least you don't have to listen to it any more! Good luck.