My boyfriend talked good about his ex who cheated. Should I be worried?

We are several months (8) into the relationship and I feel it coming to an end. He’s tired and no longer putting in effort and constantly depressed about something. Early on when we first started dating we talked about our exes to get to know each other. He told me I was quiet and shy like his ex and told me to her name to look her up on Facebook. They were together for a year, last time they talked was 5 months before I met him. The whole time he never said nothing bad about her. I even asked if he missed her and he told me no that he could get her back anytime if he wanted to. He compared me to her a lot but that was months ago. Months later he hasn’t talked about her since. I brought her up again as apart of a joke thinking he’ll laugh but it made him sick. The relationship gotten worse and he is constantly depressed about something and mad at me for no reason. He’s been downplaying our relationship and not taking it serious. I brought up how long we’ve been together and he says it hasn’t been that long.