Bouddica - you learn to repeat "No thank you". Clearly, and as often as you need to. And if boyfriend/girlfriend does not get the hint, you say "I know you're offering me food because you love me, but right now, I need to focus on cutting down on treats because I want to work on being healthier. I'd appreciate it if you stopped bringing these things home for now".

We have VERY difference experiences (I was never fit before now, you've obviously been there before), but the great thing about 3FC is that we have all kinds of folks here. Given your background, you DO know what to do. So are you ready to do it? Once you make that decision, and lock it in as non-negotiable, making the changes you need to make will be easier.
Can't wait to get to know you better!