I agree with you that it's common to have ambivalent feelings and want to step back a bit to assess where a new relationship is going. However, I see several red flags in the OP's post that lead me to believe that there are other things going on.
After 18 months of separation, he still seems to be hanging on to some feelings over his ex. I personally think he's using that as an excuse. He had plenty of time to process the break-up of a relationship where he was largely single anyway. He says he was really starting to enjoy being single when the OP came along and he started to feel 'overtaken.' I'm assuming that he means he was overwhelmed with his new status as a part of a couple. He doesn't seem fully committed to her. He says she "could be" the one and doesn't want to mess that up if she is, which tells me that he certainly doesn't think she definitely IS the one right now.
It all screams of "I want to try being single for a while and see where it leads."