They would have to decide that you weren't mentally fit enough. You would get a discharge if you were no longer capable of doing your job. When joining the military, whether draftee of volunteer, you sign a contract. The only way out of that contract is to be unable to perform. And they decide that, not you.

So if you legitimately couldn't go on, and you started crucifying mice and rubbing their intestines on your penis or something, they would kick you out.