@yukinoh1989 I think I found (at least one of your) problem(s):
You’ve got Dynamic Topology enabled, together with Multiresolution Modifier.
This doesn’t work! (See warning!)

I will send you a ‘revised’ .blend file, see if you can open that….
Your poly count ain’t that bad, so my guess is, that that’s not the main problem.
My internet is really slow so it might take some time to upload the file to Dropbox, but just be patient.