Grow up. Do your homework. Find another girlfriend. <br />
Go to https://dailydbt.wordpress.com/tag/dive-reflex/ It is a Dialectical Behaviour Therapy that acts like a reset button on a computer. It stops the spiralling thoughts. Also, set an alarm for 5 minutes. Make yourself busy. When the alarm goes off set it for 10 minutes and make yourself busy again. After you have done this a few times the feeling will have diminished. You can message me anytime.
That's funny you say no one can help you because SURPRISE! Someone can! Go to your school counselor, trusted adult or hotline. Pretty easy, and no, it's not "easier said than done". Hike up your skirt and ask for help. And also: there's billions of people on the Earth...you aren't alone! And if you mean emotionally, I'm sure you have at least one friend, family, counselor, anyone who will have any sympathy or kind words for you! And why did you break up with your girlfriend if you are supposedly "all alone" now? Think things through, kid, before you say them.
So, if you cut that should hold you for awhile. Get used to things like this that happen in life. Relationships will come and go. Instead of cutting do something else. Maybe go for a walk, use some energy, make yourself tired.