Mirror Modifier problems

I am new to 3d modeling and have a slight issue with the mirror modifier. I cant seem to keep it active throughout a character build.I will get it set up click out of option mode and lose it.I enable the cage and press the apply button, still cant figure what is going on. Any help in regards to this would be most appreciated.

im having a problem with it as well
ill apply the mirror modifier when sculpting, yet it wont do anything
if anyone has an answer It’d be great!