think of it this way, its just exciting at first, this excitement is confusing and can become sexual, after a while the excitement dies down and so does the sexual excitement, I think some of this depends on why you first wanted to try diapers, for me the diaper goes back to presexual days, my first memories are ones of comfort, this made me feel safe, the sexual side came later in puberty and it has its nice parts its not why I wanted to wear diapers, its not why I struggled with this for years, sometimes the orgasm was like a relief to this excitement, and the post orgasm would cause me shame.
I think the answer really is becoming accustomed to this as a lifestyle, that diapers are just what you wear and not some kink prop.
I use to still get erections at diaper changes when someone else changed me, I think the last time I didn't even get an erection, it was just a diaper change, by a very sweet daddy type, one that had given me orgasms in the past by hand during diaper changes.
Its just like sleeping in diapers, it takes a little practice to relax and calm down sometimes.
Also there is nothing wrong with feeling sexual excitement from this.
