There have been so many supportive replies and I do wonder whether I have much to add... But your post hit home to me too.
Like yours my Mum was always on at me for my weight. Once she bought me a whole new wardrobe as a present... It hurt so much to see every item was two sizes too small. Actions and comments like these erode our self esteem.
I am 33 now and these ideas still follow me however I've found things that help me so perhaps they will help you too.
1. Is it about you or is it about your Mum?
Look hard at your Mum. Look at pictures of her over time. Has she changed? How does she treat her own body? Is she happy and content within herself? For me eventually I realised Mum was pushing her own problems onto me. It isn't fair or ok but it does help me to accept her comments are not just about me.
2. Be quietly honest but speak bluntly. Lashing out or getting angry just gets you in trouble I found. Saying nothing lets the insults fester. At the moment your Mum says something cruel reply quietly, politely and usings the "I feel"approach.
Mum I feel like rubbish about myself when you say things like that.
Mum could you please stop commenting about how I look... I feel put down and ugly.
You get the idea. The I feel thing works because it isn't an attack.
3. Accept feedback only from people you trust who treat you with respect. There are ways to tell someone youre worried about their health without being cruel or hurtful. If people don't bother their opinion doesnt matter.
4. Look for even one thing about your body that makes you feel good and embrace it. Ive never been coordinated or sporty. But my Mum made anything physical I did all about losing weight. It made me not want to even try because it wasn't about fun but about looks.
In time found gardening and using the crosstrainer to listen to music. These things are for ME. Because I enjoy them not exercise to lose weight. If anyone comments about weightloss I cut them off. No. This activity is MINE for ME and it has nothing to do with weight. I'm firm on this.
5. Seek out healthy male feedback from trusted adults. It sounds weird but what I truly needed was for my Dad to reassure me that noone is allowed to treat me like rubbish or make me feel worthless. Is your Dad in the picture? Can you speak to him about your self esteem and the mixed messages you get. Ask him to take you to a doctor if your weight is unhealthy.
When or if you can I would love to hear back from you.