Do guys really move on?

Im with someone, he’s really great in everything. But he was in a long term relationship, i cant help but overthink if he’s really over her. (Inever been in a relationship with someone b4 him) So it always crosses my mind if he done that or this w her? did he enjoy it more? Is he doing w me what he used to do w her to remember her? He says he wasnt happy and it was really really bad and he regrets it and etc but he wouldnt breakup cuz she had severe mental issues and he was scared for her So men w long term relationships, do u really move on?