I love him but don't want to have sex with him

UPDATE: no I'm not on birth control and im almost never upset or anything usually pretty happy and upbeat never really stressed either so I don't think I have depression, the only thing that's really changed is I put on  like 30kilo (60pound) over the years but I always thought I was fat even when I wasn't so I don't think it's because I don't feel good about myself 
Okay so let me start off by saying I LOVE my finance and I want to spend the rest of my life with him 
We have been together for 4 years now when we first got together we fucked like rabbits at LEAST once a day 
Then when the new relationship hype went away we still had sex every other day so a few times a week but about a year into our relationship our life life just died and it's all on me I'm just never in the mood anymore but we wanted to have a child and I pretty much just forced myself to have sex and was miserable 
He is the only person to ever make me orgasm (besides myself) so it's not because he isn't good at it
I don't even like to fuck myself when I want/need to get off i litarally just use my vibrator on clit so I cum and then that's it I'm done, 
I watch porn I get turned on by it but then when it comes to actually doing something I don't want to 
Really need advice or help or anything because this is tearing our relationship apart and I don't want to lose him