What I take is good news on two fronts:
1. She is not going to ditch you as far as she can predict
2. She didn't rat you out to her daughter, thus she was not being dishonest.
I hope that reassures you or at least put your mind at rest?

I agree in that I believe if you had someone close to you, emotionally, you would probably not have this gaping need directed at her. But you have unmet needs and she is this loving caring mother-figure... Thus, you want to cling to her and fear losing her, so she needs to be in sight. Kinda like babies and object permanence: is mom still here, has she disappeared on me...which causes them much distress etc. maybe that's why you have this need or urge to check (is she around, real, does she still love me...?)

And yes, love is not about how someone looks but how/who they are. If you want to lose weight, why not try to implement *one* change? Ex: drink 8 glasses of water per day or replace one 'bad' habit with a healthier one or go for a 20-30min daily swim. You decide where to start... Just change one thing and keep at it. Once it becomes a habit, introduce something else.

Point to ponder (I may be wrong, if so please ignore): do you think you resent her being thin and pretty because you fear she might ditch you? As in: what can she see in me, I don't deserve her, she will judge me (as not being pretty enough to hang out with her)...