Sex troubles in relationship

Hi. I am in relationship with my girlfriend for almost 2 years now. We have a problem with sex. She gained some weight but i do not notice that and didnt point it out. She notiched it. Now for almost a year she doesnt want to have sex. We have it only when I start and a lot of times she gives me exuces not to have it also she wants to do it in the dark with lights off. Before that we had normal sex life for 20ish couple. But now its not a lot of sex (only few times per month, sometimes even worse). What can i do to help her? Will talk even be good thing to do. There is no option for us to split that is for sure. Is it unfair from me to tell her my thoughts and what I think. It is her body I know and respect that but at the same time she gets everythink from me at least i think so (emotional support, talk...) so what do you think. Please give me advice how can i help her to be more confident and if you ran in same troubles how did you solve it?