Not to sound like an asshole or anything, but it's not like you weren't warned. Let it be a lesson to you that life is never black and white like that, and no one is free from consequence; Especially not this Nick character, who will now be simply referred to as "human filth" (or HF for short). So not only did human filth lie to you, take advantage of you, and hurt you, but he did this all behind the back of a girl he's supposedly "dating"? I'm sure the poor girl in question would like to know about that. If you want to really stand up for yourself, tell that other girl everything, and I mean everything. The fact that he approached you, the fact that he lied about having feelings for you and wanting a relationship, the fact that he flat-out asked you to have sex several times, the fact that hf engaged in sexual activities with you, and the fact that he did this all while claiming to be exclusive with his other victim. Tell her. All of it. Everything. Not only would that be the honest thing to do, but HF deserves it. Well, HF deserves brain cancer, but there ain't much you can do about that but hope. Seriously though, tell this other girl. You're both victims if you don't. Don't hope karma is real, fucking be karma.
I knew that the first thing this kid was gonna do was tell a bunch of people, and that you'd end up with people treating you this way. Don't let any of it get to you, just rise above it. Be a bigger person. Whoever participates in that sort of harassment is just trying to make themselves feel better about their own problems. Whether you'd like to believe so or not, everyone has problems/insecurities, and everyone experiences struggle. How we learn from and cope with our struggles is what defines us. Like the picture in your sig says, "What matters most is how you see yourself". Right now, fuck what these other people think, and learn to love yourself. This is your life, and it's up to you to make the most of it. If there are things you don't like about yourself, try to change them, but don't be discouraged if it doesn't happen overnight. Gradually, time and experience will change you regardless of whether or not you make an effort. Whether that change is for better or for worse is up to you. You can either let your problems take control of you, or you can take control of your problems. You're more than capable of taking control, and overcoming the obstacles life throws at all of us. If you don't believe that you're capable of those things, at least believe that I do. I know I'm not the only one who does, either.
Don't blame yourself for anything.
Don't let it get to you.
Just admit you made a mistake, learn from it, and keep your head up. I promise you that if you stay strong, things will get a whole lot better. It's not always easy, but rarely does any good thing in this world come without a challenge. Trust me, I've found myself in some really dark places over the years, but as time goes on, I become stronger in spite of my problems. There was a time when I didn't think I could cope anymore, that I had no choice but to either live with the anxiety and depression, or give up. And I tried to give up. On three separate occasions I tried to take my life. I know what it's like to feel alone, I know what it's like to feel undesirable, I know what it's like to feel like a fuck-up, or abnormal, and I know what it's like to want to die. I've dealt with depression and suicidal thoughts and tendencies for a long time. And while I still fall on hard times here and there, I'm determined to make the best out of whatever life has in store for me next. There's no real reason not to.
The same applies to you, or anyone else. You still have a lot to learn, a lot to see, and a lot to do in this world. Chances are that a few months down the line, you'll find half the things that once troubled you are a thing of the past. Always hope for the best, but always be prepared for the worst. That's about that last bit of advice I can give you.
To sum it up: grab life by the horns and fuck it up the ass. Life is infinitely more beautiful when you realize your true potential.