I got my retainer today, and it's painful to put in! I only have to bottom one though because they messed up with the mold for the upper one. But I will push down hard to make it go in, then theres a sudden snap! and it goes in, hitting my gums and making them sore for about 15 minutes. It's also a little hard to take out. Is this supposed to happen? Will I get used to it?