Fiancé upset over lack of sex

Lately my fiancé and i have not been having sex maybe once a month sometimes longer. I am the one saying no every night. I've gained a lot of weight recently (I'm now passed 350) and I am just not in the mood for sex ever. I am so embarrassed. It feels like a chore and I was trying to fake it for awhile but I can't anymore I get really upset. What can I do to get back into it. I hate that he is frustrated and he is being very patient and trying to be understanding but he keeps thinking its him. Not even a year ago we couldn't stay off of each other. I don't want to go into our marige like this. I love him so much I want to be able to show it. Anyone going through this or any words of advice is appreciated.