General

Hello everybody! I have a question that has a relationship with this fetish. Is it normal that if I want to write a story I think so much about fattening my fictional characters?
I don't know if that happens because I still don't confess to my boyfriend about my fetish or if I'm repressing my "wishes", idk.

Not at all.

If you'd prefer not to tell your boyfriend at the moment, then writing stories might be a good outlet for you.
It's creative, no shame in that, so go for it I say!

It is totally normal, and as already said it is a great outlet for those feelings.

It can also let you explore these feelings in more detail in a safe way. That is there are things that you know you like (the character getting fatter), but there are things around that which maybe you are less sure? For example the character being encouraged to eat beyond being full just to get fatter, or burping a lot, or being teased for getting so fat. When you try writing things like that it can really help you figure out how much those appeal to you.

As a suggestion, don't try to put all your feelings and desires into one character and story. Making yourself write some other short stories or scenes might make it easier not to obsess as much about one character.

I can't say what normal is, but it's not strange or weird to want an outlet for your desires, especially if there is no real life outlet.

And of course you can do things in fiction that you cannot in real life, so that has an attraction as well.

Write your story and see how you feel afterward.