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

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