There are two independent issues:

1) the issue of the guitar that remains in his possession
2) the issue of his confusing you with his texts in which he says that his life sucks.

1) I am sure the guitar is valuable and needs to be retrieved. Do you have a friend (girl or boy) who could accompany you to retrieve the guitar from his place, in a casual fashion? Having a chaperone would be really nice, because it would preclude the possibility of one-on-one interactions between you and the guy.

2) Text back to him saying that you are currently not interested in the quality of his life (sucks/does not suck), but if, at some point, you become interested in the quality of his life, you will let him know. This way the ball will be in your court and you would not be severing the connection altogether, but just taking a (possibly endless) break.