Welcome to the forum. I know how it feels to be overweight as I struggled with this for many years. I eventually lost weight and feel so much better. I even needed to buy new clothes for my smaller self.
It's good your grandma is so supportive. You do need the support of those around you. Now you also have your mom which is good.
If you tell yourself you have wasted your life until now you begin to believe it. It's unfortunate you did not do well at school but you can succeed from now on. This is the time to concentrate on doing well. Occasionally it's good to look back and see how far you have come. We often surprise ourselves by what we can master when we thought something was impossible to learn.
Learning does not stop after school so if you want more formal learning you can apply to a university. Perhaps apply for on an online course though of course you miss out on the one to one sessions with your tutor and the friendship of others in the course. It's the same as your weight. You can lose some now if this will help you to function better. Perhaps you can see your GP and start getting help there. Make an effort to walk daily and/or join a gym with supervised exercising. You probably need someone to guide you through an exercise program and gradually increase the difficulty. A year down the track and you will not know yourself. I see you are an INFJ personality.
This means you are an Advocates have an inborn sense of idealism and morality, but what sets them apart is that they are not idle dreamers. These individuals are capable of taking concrete steps to realize their goals and make a lasting positive impact.
With those characteristics you have a head start on living well. As part of this personality you want to do things to help others. Have you considered a career in this very broad field? Again - they have very strong opinions and will fight tirelessly for an idea they believe in.
What do you believe in? Looking forward to utilising this part of you will help to stop your feelings about wasting your life. At 21 you have barely started your life. I see this personality likes to get to the heart of a matter. No better way than by engaging in work that satisfies your personality.
Please post in as long as you find it helpful.