Literally, one step at a time.
I don’t know how disabling your fear is. For some, the thought of a stepladder is too much to face. If a stepladder is too much, then try a simple sturdy stool. Step on it and get used to the feeling of “that high”. Enjoy and validate that you were able to do that. Tomorrow, two steps on the ladder, or actually get a ladder.
A fear of heights is rational. Falling from even a moderate height can kill you. So have a healthy fear and respect for gravity. But don’t let that fear and respect stop you from… another step.
Before I was even old enough to mow a lawn my father had me climb onto the roof of a lakeside cottage with him with a creaky old wooden ladder. (It must have been 50 years old already.) He pulled the ladder up onto the roof with us, and from there he spanned a drop of another 50 feet to a huge tree branch about 25 feet farther up and ten feet from the house.
The ladder being too frail to support his weight over a span like that, he told me what to do. He tied a string to my belt, then had me look up—only up—and take a step at a time. He footed the ladder the whole way, and talked me quietly through it: one step at a time; take it slow; look up. When I got to the branch, he had me pull the string, which pulled up a rope. Then he had me tie the rope around the branch, and from then on and for the next 20 years we had the bitchinest tire swing you ever saw.
Have someone you trust with your life at the foot of the ladder, just telling you quietly: you can do it; one step at a time.