There is expressing genuine concern for another and then there is blatant insulting, (such as what your mother is saying to you).

Of course you are no where near being too sensitive to this, (those hurtful remarks would stab anyone). Your mom is verbally abusing you and if you can, perhaps confront her with how her remarks are making you feel, as she may not even be aware of it, herself.

I am so sorry this is happening to you. I can personally relate to what you are experiencing as my hub is extremely verbally abusive and completely unconcerned about how he expresses himself. (I know he's well aware of how he insults because I have brought it to his attention so many times, not just in relation to how he makes me feel, but how he remarks about anyone in general).

Do your best to convey to your mom how her remarks are effecting you. If possibly, record what she is saying to you. Sometimes, hearing our own selves can be the most effective way to take notice of what we say.

I agree with CindyLuWho....offer to your mom that if she's willing to attend workout with you then chances are you'd be more successful with results.
(If she has genuine concern for YOUR health and weight, attending with you should not be a problem for her. It's a great opportunity for her to bond with her daughter). It's a win/win situation for the both of you....

I'm hoping for the best outcome for you....

Shangrala