Simple answer: Find another girlfriend and move on.
Getting to do that is not as simple though.
What is the best way to forget your ex? I'm kinda getting irritated and thinking about my ex gf all the time?
I wish there were a simple way to forget about an ex, but unfortunately there's not one tried and tested approach that works for everybody; everyone is different.
It is exciting meeting and flirting with new people, and if it goes any further then the attention is a good ego boost, and starting something new is always exciting. The problem I have with this kind of approach is it doesn't really deal with the negative feelings you have relating to the previous relationship, and if things don't work out with the new person, you're left feeling even worse.
As much as it's not really practical advice, I would just try and remember that all of the old clichés are true: Time is a healer, and what doesn't break you makes you stronger etc etc...
You will get through it and stop thinking about her, and in the mean time I would put all of your focus into other things, like your relationship with your friends, school/work, and any interests that you have.
Find enjoyment in other things, with other people, and you'll find yourself thinking about her less and less.