the reason is only important if you must know it. often ppl will want to know what went wrong, not to fix it but to avoid the same mistakes next time. he probably didn't explain himself did he? when you and your (ex) got back together the previous time were you just hanging on to the relationship? was it stagnating or sinking?

if this seems the worst thing right now, when you look back a few years you'd feel different about it. take it as part of living. there are no successes without failures. have no fear.

perhaps it is time to move on to, as you put it, a new life.