Gaps in between walls? ( FIXED PLEASE IGNORE )

Hello! I’m still relatively new to nomad and 3d modeling all together. I followed a couple of tutorials to make a mask for cosplay, but when I went to make it hollow and trim it to allow for it to be worn, I found that there is a gap between the walls that is meant to be filled in. I have no clue why this is and the Internet doesn’t seem to have an answer for me. I’ve tried the fill holes function and voxel remesh, but it just fills in everything instead of only the outer gap.

For clarification, I only want to fill gap in the orange part, I want the other part to remain hollow. Thank you in advance!