Tulip,
Hi there....so sorry you're going through this. 19 seems like yesterday to me and now I"m 37...yikes

I've also been having repeat ASCUS neg HPV results since June 07. So if I understand correctly, this will be your first colpo...correct? If so, then all of your ASCUS results were from paps?? A pap is a screening tool...it's not diagnostic. A colpo will be more sensitive and it will be diagnostic. A pap can miss something that a colpo can pick up. So this may be simplistic (remember I'm no healthcare expert). If you draw a circle (cervix) on a piece of paper then on the inside of that circle start from the top and draw a continuous circle (q tip used during a pap)until you reach the middle. All the places within the circle that are blank (no ink) are places where cells could have been missed. Now, in my opinion with 3 paps behind you, this situation is probably less likely that they missed something over 3 paps. Also, your negative HPV status is more good news. There are women that were neg HPV that ended up with CIN III but I would venture that this situation is probably less common. So you've got some good things going for you that tend to point toward a good outcome.
The colpo on the other hand will wash the entire cervix with a vinegar solution and any abnormal cells will turn white. THe dr will then biopsy the specific areas that turn white. The lab will then look at those specific cells more closely. So the colpo will be more specific. When you have the colpo...ask that they take an ECC(endocervical curretage)sample as well (thats the part inside the hole of the cervix). Also, ask the Dr's opinion of what he/she saw. The Dr. should have some idea of what it looked like. Some will not say anything until results are back but they should have some idea. They may not share it unless you ask.
No matter what...KEEP THAT APPOINTMENT....STAY WITH YOUR FOLLOW UPS
Hope this helps.
Take Care