17 & Heartbroken, why would he do this?

Im 17 and he is 19. I've been with this guy since the ending of sophomore year. He was the sweetest to me and we were great together, until a year later. He's into bmx riding and he started to continously putting it before me. He would leave me here and there and told me that he needs space and he wants to make it far in bmx and im a distraction. Whenever he left me, I was so depressed. I tried talking to guy friends, and my bf would always come back and yell at me and would get so mad, etc. When that happened, everytime he hurt me he would say its my fault hes like this. Like I have changed him into a monster.. anyway, he is my first for everything :'(We have been together for two years. Last week friday I was with him, nothing seemed wrong.. it was fine. Saturday night and sunday night he ignored me and when I confronted him about this he got mad and said he was just riding around. On monday night he took a break and said he needs space and promised me he loved me and there was no one else. On wednesday he left me, and it broke my heart :( what was worse is that yesterday while I was with my friend, I saw him with another girl.. I made a fool out of myself. I went up to him and cried out for him to tell me whats going on. I couldnt stop crying. Right infront of the other girl he told me to just leave him alone, and that I hurt him to with other guys.. but he kept leaving me and I didn't like any of them. Whats worse is she was laughing at me on the side of him.. he put me down in front of another girl.. after two years. And it hurts so much. All his promises were broken.. im so heartbroken..I just dont understand how he could put me down so low in front of her like I was nothing.. he barely even looked at me. And I broke down, and they walked away together.. and how she laughed at me. I would never do that to someone so hurt. I can't stand this.. I love him. Why would he do this :'( I try to be strong. I surround myself from my friends and family.. I just find myself thinking about him again.. he wanted me go put college aside for him.. im a senior now, and I pushed aside many colleges for him.. for him just to leave me and be with someone else.. im so heartbroken.. I did so much for him. I changed so much just to make him happy.. Idont understand how this girl i saw looked like everything he didnt want me to be.. He wanted me to look "cute" ,he wanted me to be conservative, no make up,natural.. And he didnt want me to be a party girl. I dont drink, smoke, never have. and she looks like she does all of that.. It's so hurtful. I made him such a big part of my life,i feel like he took so much of me with him.. I dont even hate him, i dont hate her.. It's just hurting. I wish he would come back,

Sorry for your sorrow that you are feeling but this is just how life gets sometimes. When a guy starts telling you how you should look and act, is a tell tail sign of him controlling you because he is insecure, not wanting you to be attracting guys. Second you both are young and should be active in other things other than a relationship. He's 19 and wants to experience more out of life which is normal. No 19 year old guy wants a clingy, needy GF that depends on him to "entertain" her. Your biggest mistake was making him your whole world, making huge sacrifices...now that he is gone, you have this empty void. A lot of your actions were immature, especially making a scene, that is what causes the most damage is the lack of control. I know all this is new to you, and a learning experience, you are best to lay low, and heal your heart before it drives you crazy.

You're feelings are normal right now. It won't feel like it, but it will get better. If I was in your situation, I wouldn't contact him. He's been and idiot and has a lot of growing up to do.
If it's meant to be, he'll come back, but by that time you might have moved on and found someone else or be happier on your own for a while.

Time is a great healer (although it will seem to go on forever).
Talk to your family, your friends (and even us on here!) Talking will help.
Find things you want to do - maybe a new hobbie or club or craft or something.
Go buy yourself some new clothes or something you have really wanted for ages.

There's this thing called Karma...it will come around and it will bite him ... hard!
You now have this time to find out who you are again. You're 17 and have loads of time to have fun and be you! When you're happy with you, the right guy will come along.
xx