Don't rush yourself. Accept that it's going to take time and that you have a rough day coming up.
Deal with the day itself in what ever way you have to to get through it. Keep busy. Or take the day off work, stay in bed with a box of tissues and some ice cream. Whatever.
After the day has passed, limit your pity party (I don't mean that in a derogatory way) to a particular length of time.
Then make every effort that you can you get over this person and get on with your life.
He/she took a look at you and your relationship and thought 'no thanks, I'm going to try my luck elsewhere'. This person does not deserve a lot of your time.
He/she honestly isn't worth it. And better that you find out now than 10 years down the line when kids were involved.
This WILL get better - but don't look at the relationship through rose tinted glasses. There were obviously problems (at least from his/her side).
Try to get on with your life - in a while, things honestly won't seem so bad.
There are a lot of books out there, some people hate them, some people swear by them. I keep recommending a guy called Greg Bernherdt, but he's very good. I honestly think that 'It's called a break up because it's broken' has saved my sanity in the past.
Good luck. We're here when/if you need to talk.
xx