Hi, my name's Eyman, live in Vanouver and im mostly into card magic, mentalism and metal bending.
I preformed this effect with pencils for a while untill i found out people freak out more if i used a knife instead of a pencil (be careful ofcourse that the blade is pointing the opposite way to your mouth so you dont cut yourself..a butterknife works for me as i say "the duller the harder" ). Anyway, back to your problem...push your thumb back as far as possible until it hurts (the pain also kind of sells the effect)...and make sure when you show it's going through your cheek into you'r mouth, its comming out facing more towards them rather than your throat.
p.s if you're caught and lets hope not..say sumthing along the lines of "well you should see it when i put it in my mouth and out the back of my head" as i put the knife in my mouth almost always they ask me to stop right there...haha there is a reason they call it gross magic
hope i helped since this was my first post
-Eyman