Okay so im a guy and the guy i was talking to ended up being a relationship with someone else when he told me he hadnt talked to said person in forever and that they broke up im not sure what to do, it hurts and my mental health is spiraling down right now :/

He is in the wrong and while I dont know any pre context you should take time to look at the relationship you have with him to see if its toxic, he may have made a mistake but if somebody is hurting your mental health then they are likely not good for you

I think if it bothers you so much, then you should just talk to him, like I know it's easier said than done, but that's just reality. Stressing about it would just hurt you, they should know that you're hurting, but don't say it in a way that you're blaming them for your hurt. So, I'm guessing you were dating each other or something? If I'm wrong, we can talk about it more in the comments.