My DH is a bodybuilder who competes and I HATE it. To the point that when he was prepping this year I made it clear that I did not sign up for this kind of life and if he wanted it to be an annual thing, he should let me know as I'd have to give serious thought to what I wanted for my life in the future. I promise you that being in a relationship with a gym addict enthusiast and being in a relationship with a competitive bodybuilder are two very different things.
I train at a decent level too by the way, so I understand the hard work and dedication required. Someone like your husband who has no clue will find it even more difficult to understand. (Though, if its just about you being on stage in a bikini that's a different kind of problem.)
Bodybuilding is a solo sport but it entails huge sacrifice from everyone in your household. Christmases, birthdays, foreign holidays, days out, celebrations - all very different when you are in a relationship with a bodybuilder.
It has affected my mental health in a negative way also. Being with someone who weighs every single thing that goes in their mouth. Someone who pinches minuscule bits of flesh and is dismayed at how "fat" they are... well, as a curvy woman, it really messes with my head.
It's fine for you want to do the show. I applaud all your hard work to sculpt your body to a point where competing is even an option! I do think that you need to explain the process to your husband and understand that actually, this is going to affect him quite drastically too and he is allowed to not be thrilled about it.