The truth eventually comes out. Well done for having the courage to break it off, it isn't easy. And it has only been a month so what you are feeling (doubt, nostalgia etc) are completely normal feelings so give it time and these will subside.
Thought I keep my checklist that I made with my therapist to identify the emotionally abusive traits handy to remind me why sticking this out is for the better.
I do this too, and used to read it everyday. Now I don't need to read it at all coz it is embedded in my head the guy was a complete shit and my mind now doesn't even wander to the good 'times' so when I think of him now it isn't in a positive light. This helps the healing process.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!