Yeah I totally understand what you mean.. I fear that the same would happen to me. I was thinking "what IF she calls me someday this month? what would I do?".
The good thing is that you're thinking the right way in my opinion and if he truly shows you how big of a mistake he made and how he regrets it and you decided to have him back, then I'd advice to show him hell before he steps in once again. That's what I'd do IF she ever calls back (won't happen, but IF).