Dealing with the end of a relationship is never easy. You invested a lot of time, effort, love, and affection into this person and now it's over. At this point what do you do? You know that you want to get back to day to day life, but all you seem to be able to do is sit curled up on the sofa with a box of tissues. Honestly, that's the first step to getting over it. Let yourself grieve your loss. The breakup may have been unbearable but it wasn't all bad.
Remembering the good times is natural, being upset at the loss of those good times is just as natural. You have experienced a loss. A loss of affection, a loss of a relationship, and a loss of all that time you invested. Feel free to sit at home for a few weekends, watching silly romantic comedies while wrapped up in a blanket. It's the first step in the healing process and it's a good thing. The grief is how you know that you experienced something good and wonderful, even if the ending of it wasn't good or wonderful at all.
Eventually the sadness will fade and will be replaced by anger. Anger at what depends on the circumstances. It could be anger at your ex for some wrongdoing or the actual breakup. You could be angry at yourself for something you did or did not do to contribute to the breakup. Anger at the breakup itself is also very common. While this is a normal emotion regarding the breakup, there is a right and wrong way to handle it. At no time is it appropriate to use that anger as a justification to cause harm to yourself or someone else. Channel that anger in a constructive way. For some people this is going for a run, writing your anger down, clean out the closet you've been meaning to get to for months. All of these things are positive and will let you work off some of that anger.
While some articles, books, or "relationship experts" will tell you that it's okay to keep little reminders of your ex around it will only hurt you in the long run. Keeping those things around will only bring back the nostalgic memories more often and you don't need that. At some point you may be able to look back on your relationship with fondness, but for now it will just prevent you from moving on. Get rid of the reminders. This includes deleting his number from your cell phone, throwing out old letters and pictures of the two of you. If you absolutely must keep these things around, put them in a box and don't open it for a few months at least.
Whatever you do, no breakup sex! You might sit back and tell yourself it'll be fine, one more time for the memories then you can walk away clean. Being intimate unleashes all sorts of chemicals in your brain, chemicals that are designed to make you feel close and bonded with the other person. By engaging in breakup sex, you're allowing yourself to feel bonded to them again and it might give you false expectations that maybe they'll come back. Instead of giving into these temptations take care of yourself. Eat right, exercise, and get lots of sleep. This will make you feel better about yourself. And when you feel better about yourself, you will make good decisions that will benefit your life the most.
Finally, you need to know when you are ready to start dating again. Only you know when you reach this stage, no one can tell you when you should be ready to go out again. You'll know when you get there. It will be the day that you can look back at your old relationship with a clear conscious, know that's over, and see what you learned from that experience. You will be able to picture a day when you'll have a new partner and be even happier than before. It will still be completely normal to get a twinge of emotion upon seeing your ex somewhere. But if you still have the desire to key their car or throw a plate at their head then you haven't moved on completely yet.