I want to go for a thru-hike alone without my girlfriend
Why don't you want her to go with you?
I was raised to be independent, so I like my own space.
If it means that much to you to go alone, then don't bring her.  She should be fine with giving you space when it's important to you.  That's a sign of a healthy relationship!
I bet she will get depressed and do something stupid like killing herself.
I have to assume that you are being facetious, but just in case, if you really worry about your girlfriend harming herself out of loneliness, then this is too big a problem for you to take on by yourself.  You should get her professional help.  However, if you were just exaggerating for humor purposes, then I suggest that you simply let one of her friends know that she could use some company while you're away.