need advice

My boyfriends ex messaged him on snapchat and i had brought it up and he got mad at me .. he had told me when we got together a year ago he had blocked all his exs. But this one has popped up twice in our relationship i dont know if i should be worried or not he says he wasnt flirting but still he went behind my back and was talking to her .. whats should i do hes been acting different towards me and says he feels drained and he doesnt think he can give me 100% anymore because hes not okay mentally but he doesnt want me to leave or break up, he wants to make things work but i dont see it . I feel like he no longer loves me or wants to be with me maybe i am overreacting but i dont know everything was good so i had thought. I always asked him what was wrong when i knew something was off but he hasnt been talking to me about it . When we first started dating he said “ communication is key to a successful relationship “ but yet he hasnt been going by that for months now . I do not want to get hurt or feel like im not enough.. im at the point i just need some advice..