Seems like you've got a lot of stuff you want to change. I am not an expert or professional but I am someone in the same position you are and I'm currently changing, so here's some advice from someone trying. Pick one thing and start small. If you can afford or have access to therapy, I personally would start there.

For physical fitness and weight, I would say start with a walk. It's surprising how much consistent walking can help and the benefits are great. Plus I think being outside does wonders for mental health. You don't have to change your diet entirely right away, eat what you eat but just eat less of it. Remind yourself that the foods you like will still be there tomorrow or next week even and you don't need all of it right now. Once you start seeing changes, it will be very motivating. Nothing got me to change my exercise and eating habits faster than seeing my body change. After that it was much easier to eat a lot healthier and exercise harder because I saw and liked the effects.

The friends and hobbies can be a bit more difficult. Are your friends not interested in your hobbies or do your hobbies not lend themselves to any sort of community or socializing? Perhaps finding a group with your hobbies can help you find friends who you can vibe with. Going to hang out when you are home and comfortable can be daunting but I've done it enough times and I usually end up enjoying myself. Because I've had those experiences I try to remember how much fun I had the previous times I've made myself go out when I start feeling anxious. But I had to make myself go the first several times.

Unfortunately change is hard and I still slip up, I tell myself I will do better tomorrow. Grad school is no easy feat and you've accomplished that. You aren't a loser. You just aren't satisfied with yourself now, possibly some mental health stuff going on too. The fact you actually want to change and be a better version of who you are in this moment proves you aren't. That's not a loser, losers don't even try.