I had that problem; terminated with my therapist in 1987 and then wanted to see her again in 1996. I didn't have her "current" phone number, but knew she had been in the phone book so I went to the library, it had all the local area phone books (I lived in the Virginia, Washington, D.C., Maryland metropolitan area; I lived in Maryland and she lived in Virginia) and found the number. I called and, surprisingly, she answered :-) It was wonderful, when I said who I was, she replied with pleasure and was really glad to hear from me, "recognized" me, etc.

I would just write a little script of what to say, "Hi, this is SingDanceRunLife and I saw you X years/months ago when I was in high school and I'm getting ready to go off to start college and wanted to come meet with you again to work some more on my anxiety and OCD; would that be possible at this time?"