Hey babe, I can completely understand how you feel! Being happy for someone yet almost - dare I say - resentful
, all part of being human I guess.
I agreee with 'niknak', you are taking the right steps by wanting to do something about it, but rather than focusing on the negative side of things, turn it around and see the positve. i.e. you and your mum are both going to be so much healthier and happier (and live longer!) use your mums achievments to motivate you - chat to her about it and you can work together to lose the weight. I beleive that a good healthy diet is the key - you can do all the exercise in the world but if its the wrong thing going in it'll all be in vain.
Keep your chin up sweety, where theres a will theres a way - and if you need to chat we're all here!
p.s. I have a couple of friends doing a healthy weight loss plan that seems to be working! i'll try and find out what it i if you're interested...