Sounds like you have a global permissions / rights issues.
Let’s hope it’s only user side, and not system wide.
Please post here outputs of these commands :
ls -la /home
ls -la /home/your_actual_user
…of course replace your_actual_user by your real user’s name.
Let’s also see if there’s enough storage available :
df -Th | grep -v loop
df -i | grep -v loop
and a picture of your different partitions ( set terminal in full screen before issuing this command ) :
lsblk -fe7
These are first steps to better see what your system looks like.
Now some questions :
⋅ did you ever launch graphical applications from terminal, using the sudo command ? Things like sudo nemo or sudo gedit or sudo any_windowed_app ?
⋅ did you ever try to change anything regarding rights and permissions on some files or folders, using command like chown or chmod with or without sudo ?
We could see that with :
history | grep -Ei '(sudo|chown|chmod)'
Please post here those different outputs between 
marks, for readable formatting.
And let the terminal prompt visible ( it looks like name@machine:~$ ).