How do i deal with stress that my girlfriend is giving me?

ive been really depressed for the last 2 days considering that my girlfriend broke up with me becuase she said we had no connection:/ she said that no one will ever be as good as me and shee will never love anyone eles, so its been 4 days now and shes allready liking another guy and kissed him me and her were together for 6 and a half months and she said she wasent gonna ever break up with me but sure enough she did i really am depressed im usually the happy go lucky guy but i am not the same without her i never went threw depression before this is my first time and its really eating at me. seeing her with another guy gets me extreamly mad to the point that i hurt myself ive been pinching my skin until it turns blue to relief stress but nothing is working to get her out of my head even more worst the guy that she likes is in my spanish class and i saw him walking towards her after lunch, this really hurt me cause we did everything together and i plan on marrying her one day. so any advice of what i can do to deal with the fact that she is with another guy right now until shes ready for me, im really depressed and im only 16 and i already am feeling like killing myself over her i need adivice so i can stay positive until shes ready and that her kissing another guy wont get to me anybody have any ideas? :''(