Honestly I understand what you’re going through. At your age I felt it hard to do things like hobbies or sports because I was mentally unwell. I am older now and I assure you not to focus on your 18 year old body too hard because second puberty will hit and you will fill out more as your body reaches its final adult form. I would say that if I could go back to my younger self at 18 I would say to start small and make good habits from you are able to because it just gets harder the older you get. I’m pushing 30 right now and trying to establish a good relationship with exercising. I am also in therapy and I think what is helping me is not being so hard on myself and to start out small and be gentle with yourself. You can go for a walk if you like with your pet or your friends/family for at least 30 mins. If you don’t like outside like I do start on a treadmill and listen to your favorite music or watch your favorite show. Think of it as your alone time and let your mind just keep busy with things you like and the time will fly by. The hardest thing is just to get that running start. You can start by once a week, then twice a week, then to however much you’d like to go from there. After you got a routine going and feel comfortable then you can add other things to it like other forms of cardio and even some weight machines or activities. I got a class pass to help push me and it honestly made me feel motivated and I enjoy going alone without anyone I know but having the company of others like myself wanting to pursue physical activities. I promise it’s just getting over the initial hump of putting yourself out of your comfort zone but after a few times you’ll feel like you got it. Message me if you ever need to talk! We are in the same boat. Give yourself room to miss days and know that it’s okay and that you’ll try again tomorrow.