There are two ways that any discussion of your ex can go.
This is predicated on the understanding that the reason you broke up was the reason that any relationship dissolves. Either communication works, or it doesn't and you go your separate ways. Therefore, on those terms, it can be acknowledged that on the one hand – there were very many reasons why you broke up. And the decision to do so at the time was by no means incorrect, or not what you needed at the time.
However, times change and so do people. We would do well to remember that more often in our life. Mistakes need to be made so that we can learn from them. Who's to say that we can't learn from them with the same person. The key difference is that while it is the same human that you're talking to again, it's very unlikely to be the same relationship. You owe it to yourself to put yourself out there and take some risks.
Otherwise you won't end up doing anything
If you aren't using your heart or opening yourself to new things – or even old things – then you can't hope to reap the benefits of a relationship. This can also offer some much-needed closure. Even if it doesn't work out with the ex again – even the second time around – at least you know. No relationship is a waste or not worth your time. You learn so much from each other, and about yourself. It's unlikely that every relationship that ever ended was never your fault.
We have to acknowledge that none of us are perfect, therefore it might be that we needed to change. Not the other person. Therefore, after a certain period of time has passed or we feel more ready for a relationship, who's to say that it isn't worth giving it another go? There was clearly a reason you were together in the first place.
One exception to this rule, however, is cheating. There's a lot I can forgive in a relationship, but cheating is where I stop. For my own good.
You're minding your own business, stupidly in love with Harry from down the road. He bought you flowers last week and always cooks for you. Life is good and you consider yourself safe and sound – it never even crosses your mind that he might not have been faithful.
Or equally, it could be the one that we really should have known better than to fall for. It's always the people we shouldn't have fallen for that hurt the most when they leave because it means you were right. But we can't help it. Love is all about those moments when you're minding your own business and then you realize your heart has gone on a romp about in someone else's bag. And there it goes! Nothing you or I can do about that. No sir.
But back to the emotionally turbulent matter at hand
He's cheated. You hate the way it makes you feel and you break things off with him. But it's not as easy as that. The only reason it hurts so much is that you cared about him so much. Missing people against your will. Like it or not, your heart is still at their mercy. They may not even realize it, but that makes it even worse that people can keep such a casual, loose grip on all that tethers you to this mortal coil. I'm being dramatic but also I'm not.
However, in general terms, you need to go into relationships willing to make mistakes. To be wrong, or make the wrong decision. I'm not saying that you need to use this as an excuse for emotionally masochistic self-sabotage. But try not to wrap yourself in cotton wool at every hour of every day.
Otherwise you won't feel the rays of the sun or reflect on the memories of the past.