Being bitter is okay cause it is what people do to show they are hurt, but do not let it overwhelm you. Do not contact him / her until you are really over him / her. Avoid talking about what happened between you, but do not lie to yourself -- do not rush things.
When you're so done with your bitterness, accept your faults and shortcomings, but do not blame anyone, even yourself, for the break-up.
Know when you're ready. Find someone else / focus on other things. When you're confident enough of your feelings, then start talking to him / her and be friends again.