Do I (19F) tell my boyfriend (22M) that I used to be a guy?

I had an appointment with my therapist yesterday and told him that I’m going to propose to my boyfriend. He asked if he knows everything about me, and I said that there’s one more thing left to tell him. I used to be a guy. I had gender confirmation surgery a few years ago, but my therapist was saying that I need to be open with him before I propose. I think I should, but I’m scared. I’ve been afraid to tell him because I don’t want him to see me differently. How would I go about it the best way?