Whenever a relationship ends, it is tempting to hide the evidence. While I agree that some time and distance can give you perspective on a relationship, you should absolutely attempt to figure out what went right and what went wrong. You need to conduct a relationship autopsy.
While it would be nice, do not expect your ex to participate. If he is willing to answer a few questions, make it one session and don’t attempt to defend yourself or the relationship. Be aware that it’s highly likely that he’s going to lie to you about or at least omit the reasons why you broke up. He still wants to be the nice guy – even after the break up. Attempting to talk to him at length about it is not going to improve this, it will only annoy him and make him avoid you in the future.
You may also want to ask some of your close friends and family what they think went wrong with the relationship. Be aware that they will be unconditionally biased toward you. So don’t believe them when they tell you that it was all his fault.
Your biggest source of information will be yourself. Think back about what was happening when the relationship was happy. Was it merely because it was new? Were you still putting your best foot forward? What parts of your personality were on display? Were you being inauthentic? Did the relationship go bad after a specific event or length of time? Did you seem to be fighting more often? If so, what about? Did you not fight because one or both of you were closed of? Did you have different communication styles? Did either of you give up?
As this won’t take an evening, you might want to journal it out so you have a written record you can go over. The process may be painful but it is worthwhile. If you can figure out what went wrong in the relationship, you can avoid the same mistake(s) in the future.