There's a pretty good chance he was already a lying, cheating ass hole, and just waited to let it out. Don't let him manipulate you, dump him and move on.

Don't blame yourself. If he became that kind of person, the chances are that he was going to with or without your influence.

Gasp! you've unleashed the beast.

If it's not broken, don't fix it.

Dump him.

Tell him that. He's misusing his new confidence. If the only reason he was caring before was insecurity, then sure, dump him. But there's a chance he's just reacting to unaccustomed power and can find a valance if he understands that.

Time to ditch that monster you created!

It was kind of you to try and help him get over his insecurities... but isn't it just as likely that he was always an asshole and just lied to you the whole time by hiding his true nature? I-m not trying to be a downer, I just don't see someone having that fast of a change in personality because an (likely) unqualified individual helped them through some stuff...

Well you must've done something different than just helping him, there's more to this story that you don't say like a 4 year old about to get in trouble

Story of my life. You always think their different. You let them know that everything is okay. Them BAM they are the opposite of what you thought they were. Sorry op you don't deserve that.