I was actually feeling this way like about an hour ago, so I know what you're going through. They do that stupid **** and it's obvious they really just don't give a flying **** about you at all.
Part of it is to accept that they just don't ****ing care. They don't. Don't and never will. Who knows why? Women are impossible to understand. It will not be that way with every girl, but for many, it will. I can't ****ing figure it out, how she can care more about some prick she just met than some dude she has a long and passionate history with, but that's life.
So you accept that that's just life and move on to the next one. In the long run it really does not matter what this one girl thinks. Acceptance is the key.
But these are my definite quick fixes for when I feel like ****:
1) Crank up some music extremely loud and get really ****ing into it. Whatever makes you feel good. Personally I go with punk **** cause it gets me really amped. Last time I used How Could Hell be Any Worse by Bad Religion which worked just fine.
2) Read some PUA material to remind yourself how to act and how to live. Tonight reading
http://www.pickupevolution.com/edge-hamog/ really picked me up for some reason and most of Pook's stuff would also serve quite nicely.
3) Remind yourself that she is the one who is ****ing missing out by blowing you off. Remember she is just some dumb *****, but you are actually a rare catch amongst men. Because this is true.
4) Finally, I just keep frame control in mind. I want to be an immovable ****ing rock. I do not want to be a whiny little ***** who will cry when a girl doesn't like him. Mystery says it best, "If you don't like me go **** yourself." Constantly remind yourself of this goal. I will not let my emotions control me, though I may be a raging storm on the inside I will appear as a placid lake on the outside.
That is my answer to your "How to deal with it" questions. Just act like it doesn't ****ing matter to you one bit. I think that, in most cases, she will respect you the most for this, and you will also respect yourself the most for it. Certain times you MUST take action, but learn to figure out what a situation calls for.
5) And lastly, the golden rule: Spin More Plates! The only one guaranteed to work every time, but definitely also the hardest to execute. Do this and all your problems will just melt away.