Is there a way I can cut/file a small part of the plastic on my retainer that is hurting me?

I got my braces off years ago & wore my retainer every night until my final visit when he tightened it & it started digging in the bottom left side of my gum. The pain would be unbearable after 3 nights. I should have went back but didn't & now I'm living in a different area & wondering if there's a quick fix for this & what's the best way to do it? Also, my teeth have shifted a bit, will my retainer be able to shift them back?