don't let your doctor scare you! i had a gastro tell me the same thing, he made the tests sound really horrible, and i had been putting them off for years out of fear. i finally got them done last month and they are not a big deal.
for the manometry, the tube is kind of big, and it hurts a little going down, but it's not unbearable. once it's in, and you swallow, it just feels like a sore throat. it may hurt when they pass it through your nose because sometimes it's difficult to find the hole back there, but again, it's nothing you can't put up with. they'll push it in- you'll barely feel it- and they'll probably ask when it's in your thoat. when you feel it there, you gulp down water while the rest of it goes down. i didn't gag once.
if you're getting the ph test with the tube in for 24 hrs, that is a piece of cake. i didn't even feel the tube if i was sitting still. it burned my sinuses when i ate. the most annoying part for me was getting the tube caught on stuff and having it yank me backwarRAB, haha. i actually found it relatively comfortable. again, no gagging.
the manometry will show how well your esophagus is working and the pressure in the esophageal sphincters. the ph test measures acid and you have to press a button when you have symptoms, are sitting or standing, lying down, and if you're sleeping, eating/drinking.
these two tests are worth it because they give such a wealth of information. i was scared of them for so long and it was all for nothing. you'll be fine. good luck!