Why does my ex not just let go after he cheated?

Question

Why does my ex not just let go after he cheated?

My question is ... I'd been with my ex for two years when I found out he been cheating, when I'd met him he said everything I wanted to hear, how he'd been hurt , just wanted a true love thing with no crap no lies etc .. He seemed really sweet and genuine .. After a few months he was moody controlling and blamed me for everything.. He did the odd weird stuff but when confronted he always had the perfect excuse so I stupidly just past it over.. In January I had a weird gut feeling although we were getting on perfect ( probably why the gut feeling eh ) anyway I went digging and found out he'd been seeing a woman in the previous October , I dug deeper and she said he'd told her I was his crazy stalker ex who wouldn't leave him alone,( although I was usually just texting to see what he fancied for tea as we lived together ) he even drove past me with her in the car and waved and swore blind it was his mates mum .. She later told me that they just got back from a night away together when I saw them .. When confronted he told me I was mental that it was all in my head and that I'd never trust anyone after what my husband did , and it's sad I won't let anyone close to love me .. After getting nasty he left.. But then constantly pursued me, I stupidly let him back but after 5 more months of sulking and no trust and weird stuff I told him that was it, now he won't let go, just keeps telling me I am denying my feelings , no matter what I say he will agree not to contact me then text the next day saying ' morning gorgeous, how's you today ' he's driving me mad! Keeps telling me it didn't work cos I won't let him in to love me .. I want to know why if when he had me he cheated and wanted someone else, now he can go have whoever he wants ( and I'm pretty sure he still is ) won't he leave me alone ? Have I just become a challenge ? Is it kid and you syndrome ? Help !