Fiance going to the gym... UPDATE

I used to be 150 lbs and was craaaazy into fitness. However, I've since came all the way up to 320 and absolutely let myself go. My SO and I got together when I was about 260 and we have been together a few years. I've been dying to get back into a gym but haven't had the motivation or couldn't afford it. Lately, I've had motivation but Planet Fitness needs me to pay outlandish past due fees. Well today, my dream gym (RetroFitness) is having a cyber Monday sale. They dropped the $200 enrollment fee and I owed nothing today. I was super ecstatic. I can finally get myself back together! And they have free childcare so theres NO excuse. I tell my SO and he gets super excited and wants a membership as well. It's $35 per membership. He wants us to go together and get in shape together. Hes super excited too. For some reason it made me feel a little off. For a few reasons and I can't put my finger on just one. My first reason is I don't think he'll go. And I don't want to pay $70 a month when it's a year contract. My second reason is I feel like it's sort of my thing. My time to get away from everyone and have some alone time. My third reason (and I'm preparing for you all to jump down my throat) is jealousy. Cold, hard jealousy. I've gained so much weight and there's bound to be women there with ridiculously good bodies. Beautiful women. And I'm just kind of the fat girl trying to lift a 10 lb weight. I don't want to think about every pretty girl I see and if he's looking, I want to think about my work out. We don't watch porn or really look at others like that. We have always been really secluded to each other and our family. We don't even have friends, but we thrive this way and we are happy. I guess if I'm being really honest with myself it's like..I don't want him to be around all of these fit women all day and find me unattractive. I know it's irrational. Does anyone have any advice to get over this? Maybe some alternative ideas I can give him to keep him away from this gym thing?

UPDATE: I know I'm being ridiculous. I don't need to be told. I can't change how I feel unfortunately. So I'm asking for KIND advice as to maybe how to move passed this. I know it's irritational but I can't help my emotions.

UPDATE: I also forgot to add that our finances are 100% joint. There is no "I pay for this" "he pays for that". We have 100% joint accounts and don't pay attention to whose money is whose.