Soda and the likes would definitely be a strong hinder - so many empty calories in these, they don't even make you feel like you have something in the stomach, it's like, well, it's all useless, so to say?
And I'm going to be a little graphic here, but regarding the fiance bit, just think of the intimate moments to make you feel courage.

I like being on top, and he likes me to be on top too; I don't want to gain weight and end up making him physically uncomfortable under, uh, said extra kilos.

Perhaps you could think about similar things to help strengthen your resolve? (Oh, and be aware of quantities! We women tend to easily end up eating as much as our men, and it's noooot good at all.)
You don't have to start like a powerhouse and do "everything right, right from the start". For instance, walking only 15 minutes a day for starters doesn't take much time, and is a nice form of exercise that can easily be increased once you feel better and "trained". Cutting down on one soda a day, then o ntwo, etc, would be another example of what can be done. Baby steps are easier to follow on the long term than big, overhauling changes that make us feel like it's impossible to do it, and like it's too much.
In any case, I hope that you'll find what it takes for you to go on the healthy road. It IS worth it, I can tell you.
