If you feel like crying, cry. If you feel like eating an entire pizza, eat the entire pizza. If you feel like sleeping all day, sleep all day. Whatever you feel like it will make you feel better you should do (as long as it's not something dangerous).
Let yourself mourn this loss and feel whatever you have to feel at the time.
You will heal a little more as every hour, day, and week passes, don't pressure yourself.
Remember you're not to blame about this, neither is he, sometimes it just isn't meant to be.
