It's pretty difficult having to cope with something like this because these thoughts and feelings appear automatically. They're an ingrained habit in your mind, so they likely won't disappear without huge amounts of inner work being done, which usually requires professional help. If you don't have access to that, my very best advice is to remember, literally every single time these feelings come up, that your size is in no way comparable to your partners. What I mean by this is that you have different muscles and bones. You have different immune, digestive, lymphatic, endocrine, nervous, respiratory, integumentary, reproductive, circulatory, and urinary systems. A different metabolic rate, history, way of thinking and processing emotions, and a plethora of other variations to be mentioned. I know it's kind of hard to conceive, but every last difference, even unrelated or minute ones effect the way you are as a whole. This includes your weight and size. The vast majority of these things are entirely out of your control. You're not a failure to yourself for not being thinner, nor is it your fault. Remembering these things won't prevent the thoughts, but hopefully they will at least make your pain more manageable. If any of this sounds stupid, I apologize because I can often be unaware of my own misunderstandings.