The best way to "get over" a girl is to work on making yourself someone that other girls will feel attracted to. More importantly, it'll make you into someone that
you feel like girls will feel attracted to. You need to find something to distract yourself from thinking about all the stuff that brings you down and focus on doing things that'll bring you back up - Both in your own eyes and in the eyes of others. You sound like you've got some self confidence issues, and that's alright. Once you admit it you can take steps towards fixing it.
I know you're asking for advice on getting over a girl, but bear with me. A lot of "getting over" someone is becoming a better person and learning from the experience. Not everyone you reach out to is going to reach back, but you should do everything you can to be someone that folks are willing to reach out to. Think of the kind of girl you want - Not specifically
her, just some general idea of the interests, looks, and personality a girl you see yourself being attracted to. Now think of what she'd be attracted to. Try to become that person. If you can't, it's time to reevaluate your expectations. A lot of guys dream of a girl that's going to show up and "fix" him. If you have that daydream, throw it out the window. Maybe go outside and bury it for good measure. Those girls only exist in Hollywood.
So here's what you do:
You gotta work on your appearance.
Exercise regularly and with energy. Spring's coming up, so this should get easier as the weather warms up. Don't feel nervous about going to a gym; as long as you educate yourself on proper exercise practices no one's going to make fun of you. In my experience they're more likely to come over and give you advice.
Get a haircut, and figure out how to style your hair. This means taking time to use conditioners, combs, and styling gels to make it look nice and get it under control. Even if you decide to let it hang you should do what you can to make it look nice. Handle any other appearance issues that you have. Google is your friend.
Find some clothes that look good on you. If you're skinny, you may need to special order something from a website. A lot of clothing companies have thin fits, but they don't carry them in-store. If you're tall, the same companies usually have tall variations, but they tend to cost more.
Here's a handy guide for tall/skinny folks. If you're not tall and/or skinny, lucky you! You're all ready to start building muscle and don't have to worry about special order clothes, and discount options like thrift shops and clearance racks are viable options. Either way, read up on modern style and figure out what works for you. If you have a friend who's moderately familiar with fashion, ask them for advice.
Expanding on that, keep your house/room/apartment/whatever clean. If you've got any control over the decorations, it's time to think about what a girl might think when she sees them. I used to have about twenty
figurines statues and posters of dragons spread around my room. Leftovers from my childhood... ten years ago. One day I realized, "Wow, if hell freezes over and I get a girl in here, she's going to nope her way right back out the other direction." Try not to have more than one or two decorations that might make someone think you're less mature than you really are.
Next, you gotta learn.
Eat healthier. Learn to cook well. More than just eggs and sandwiches, man! Learning to cook is amazing, because making a good meal for a girl is great 3rd or 4th date material. Learn some basic meals, get a crock pot and learn to appreciate it for the wonderful tool it is. Worst case scenario, you'll be eating better and packing some delicious meals.
Develop a healthy hobby. Draw, write, paint, design games, learn guitar, whatever. The important thing here is that you're doing something that's primarily creative at its core but physical enough for you to appreciate. Whatever you decide to do, you'll probably suck at first, but every artist has a thousand bad paintings/stories/songs to get out of the way before they finally get to the good stuff. Don't let the journey wear you down. Practice your hobby daily. Everyone likes someone who's creative, but it's a muscle you have to work like everyone else.
Gain a skill. In the world of the Internet, this is incredibly easy. Programming, IT support, and just about anything you can think of is documented thoroughly somewhere on the Internet. If the Internet fails you, start looking into universities with decent online courses, or local universities if they have the courses you want. Eventually you'll have a skill you can market, and if it's something like Programming you can fiddle around with it in your spare time and maybe make a couple of bucks that way. If you already have a marketable skill, great! Become an expert in your field. You want to become such an asset to whatever company that wants to hire you that they don't dare dream of losing you. Having a steady, well-paying job is attractive in its own way, but you're going to have to put effort into whatever field you decide to go into.
Don't procrastinate because you're getting a late start - A late start is a heck of a lot better than no start, and from the sounds of it, you aren't even in your thirties. You've got a long life ahead of you. Work hard and learn stuff so you'll have a better future, even if it's only going to pay off a few years from now. Life is an investment, so you're going to have to make some deposits.
Finally and most important of all: Love life and love what you do. Love what you are, and become what you love. Self confidence and passion together are more attractive than anything physical, and you'll be a better and happier person. If there's something that makes you unhappy in life, work towards fixing that problem, mitigating it, or learning to deal with it emotionally. It's not going to go away on its own and you'll be a better person for it.
Do your best to put your feelings for her on a shelf. You may never be entirely able to erase your feelings for a girl, but you can mitigate the problem by moving on with your life and making yourself into someone that both you and someone else can love. Look at it objectively. If she really is a bitch, do you think a relationship between you two would work out? Could you be happy in such a relationship? Unless they're gunning for the girl, your friends are looking out for you. Don't make the mistake of distancing their advice.
Best of luck.
Edit: Sheesh, sorry that came out so long. I'm not very good at short and sweet.