I'm sorry. I should've told you what he's like. I was in your shoes, yet instead I watched you be his next victim. It might be too late now, but know that whatever he said is not true. You were not the reason.
When he said it's in your head and that it's your insecurity, know that he meant you're close to catch him. Your instinct was right; he was gaslighting you.
When he said it is your fault that it's over, know that he meant it is your fault for realising that you're not the only girl on his chat list. Your gut was right; you deserve better.
It might be too late, but I hope this give you closure. You're not the problem, it's always been him.
To his new girl, I hope you can escape being his next victim.