How to start a difficult conversation?

Okay so I've been with my boyfriend for about 8 months and lately it seems like he quit putting any effort into our relationship. I want to talk to him about what's going on or if he's unhappy in our relationship or something else, but I have no idea how to start that kind of conversation. I don't want him to think I'm breaking up with him but I also want him to realize it is a big deal. I'm extremely passive and things don't usually bother me so I usually never have anything to complain about. I'm 23 and feel completely stupid for asking this question but how do you start a conversation like this?