Hi Joyce,
I understand what you mean. I have been in a relationship for 3 years a while ago, and during that period we broke up and got back together 2 times. (We broke up permanently half a year ago.)
What I've found, is that when you get back together after being separated, the relationship is no longer the same. For me, I kept on thinking about what happened during the break-up and kept on seeing this as a "stain" on the relationship. It would just never be the same again, never would this relationship be pure and all good again. It would always have this bad episode written on it.
You try, of course, and you attempt to forget it, move on, be happy again... but every now and then it comes back, and it can hurt a lot when it does. It can strike in the most peculiar moments, but mostly I've found is that you end up bringing it up during fights or arguments. Which of course doesn't make matters better...
Initially I was the one breaking up, but I was also the one regretting it the first time. I can tell you, I didn't have a "good" reason either, as you put it. Sometimes you just don't need a reason. You just feel that you are not in love with the other person anymore, or just don't feel like you want to be together anymore.
I think it would have been better for us, if we would have stayed separated the first time. Like I said, I ended up regretting the decision, missing her. But when I reflect back on it now, I think I just missed the idea of her, the idea of how our relationship used to be. Maybe I just missed being with someone, anyone.
So although a break up is painful, like you say, when you get back together, for one side there will always be the fear and anxiety... will it happen again? Will he/she leave me again (for no reason)? And for the other side, the guilty feelings of having hurt the person that loved you, the fear of doing it again...
I think, a break up has a reason (even though one of the two may not see it as clearly as the other) and you should not forget this. Then, if you do think about coming back together, you should think about if it's really worth it to possibly go through all the pain again later, and to have the fear and anxiety, just for the sake of some extra happy days/weeks/months.
That's my take on it.