All of a sudden, my girlfriend says she’s no longer happy with me and started to want to change who I am. At the same time, she started hanging out with her cousin more, who has just become single. He has many guy friends, which my girlfriend had previously dated.
Every time she goes to hang out with her cousin, and she tells me they only hangout at his apartment, she gets fancied up. But no longer does that for me. I’m trying to figure out if I could trust her now, in the past I could, but recently not as much, due to her wanting to hangout with him more when her ex-boyfriends are around. Can I get some insight?
LemonVibe 
If she’s dolling up to go to someone’s apartment, I’d begin to worry. Maybe go with her to her cousin. See what it’s all about. See how she reacts to it.
But no, she shouldn’t be hanging with her exes and using her cousin as an excuse. If it’s not a problem, then she’d happily invite you/let you tag along.
If she weren’t doing something out of the ordinary, I would say you were being jealous, but since she is, I think she is finding things wrong with you to eventually have a reason to break up. You can go along with her, but honestly, that will only slow down the process.
I would talk to her regardless. Relationship issues are never “all of the sudden” and instead of talking and fixing the actual issues, she’s on the prowl for someone who doesn’t have the issues (granted does have other issues or else they wouldn’t be a ex.) Communication and meeting in the middle is key if you want to save this relationship. It will take both of you however. If she’s not doing her part, move on and don’t waste time on drama when you could be finding the someone who will be a better person for you in the long run.