Singing Advice

[LikeAWillow]

I know that we have some absolute fabulous singers on here, so I thought I'd turn to you guys with my little problem. I've been singing since I was in elementary school, in various choirs, festivals, select groups, yada yada, and when it comes to singing melodies and lead vocals, I'm pretty good. But oh god harmonies are my downfall. If someone gives me just the first note of any harmony part, I can sing it perfectly in pitch the rest of the song, but without being given that note? Oh, you don't even want to know how bad I am. I'm just not able to hear the parts the way I'd like to be able to, and I was wondering if anyone here had some advice?