First be grateful for what you've got and proactive on what you can change. You're tall ( which women love ) and can lose weight and get in shape. I'm a lot older than you. Try going though life 5' 1" and bald since 20 then you'll know how it feels to be made to feel almost like a leper by women. It's only in the last year that i've met a few women and got a bit of confidence. I'm chatting online to a women and so far so good but whether she'll still want a relationship when she sees me who knows? My self image and lack of self esteem is probably just as bad as yours but you have to make an effort and put yourself out there.
Staying in 99% of the time and working out on your own ain't the way to do it mate. Go to the gym and try and get a gym buddy. Don't dismiss online dating and like illegal toilet said it doesn't have to cost money. Sure it can make you feel like crap, but there are decent people on there and like they say 'you gotta be in it to win it'. You just have to develop a thick skin.
Get out and do stuff. whatever it takes get outta the house.