Tell bf I’m losing interest

How can I tell my bf that I’m losing interest but want to stay in the relationship? I met my bf my freshman year of HS and we’re going on 9 years now. He’s planning on proposing soon but it’s worrying me that I’m slowly losing interest. We bicker like an old married couple, and trust me it gets so annoying. We have sex once every 2 weeks or so and the same moves are becoming boring. Sometimes I have to fake an orgasm just to get it over it. And he’s been gaining a lot of weight and I’ve suggested he go out on runs with me and cut back on junk food but he isn’t committed.

But I truly love him and want to build a family and a home together. We’ve been through a lot together and I want to make this work but idk how to go about it without hurting his feelings or ruining my relationship. And with all the bickering, the intimacy issues, and our communication not being all the way there, it’s affecting my relationship.

Any advice on how to talk about these issues. Ways to say things nicer? What I should and shouldn’t mention?