How do i tell him?

I don't know how to tell my fiance I have depression.. we've been very close friends for 5 years before getting together two years ago, he almost knows everything about me.. Almost. The fact is that I've never been open about my mental health and never told him, or anyone else, about it. he knows that i went to therapy for a while but he always thought i did it because my parents wanted it(I was underage at the time) and i let him believe so. I am unable to talk about me and my mind, and i always felt ashamed of my problems(my father does not believe in psicology and is an asshole with me about this). Now I don't know how to tell him. I know that I should, and that it will not change his feelings for me, but in my mind it feels like I had to tell him i killed his mother. We just bought a house and are about to move and build a life together, I no longer can keep this "secret". Do you have any advice? How can i open up about this?

sorry for the poor english but it is not my native language 😅