Any ideas on how to overcome a fear of Heights?

I have always had a fear of heights. The first time I rode a rollercoaster ( at the age of 23) I cried all the way through the line. The first time I flew, I had to take a valium just to keep myself calm and couldn’t sit by the window… Those 2 things I have overcome I guess, I have to fly often now and after the first roller coaster, they were really fun! I’m usually ok going up stuff, but the coming down part freaks me out, I freeze up and start shaking, get a case of vertigo sometimes. Any advice on how to overcome this?