Hi Puddin,
I would say that if you're feeling depressed by the journaling topics then you've probably picked some good ones. Remember the goal is to uncover the buried emotions, if you're feeling depressed then you're obviously responding to them. In other words you're feeling, which is what you want if you want to recover.
But I know what you're saying, depression isn't pleasant, and for someone who isn't used to it, it may even be frightening as well.
If you're worried about going through a one-way door into depression that you can't escape, know that all emotions are transient and impermanent. There's always an escape.
Why not allow yourself to create space for your depression while you're journaling but immediately afterwardsues do something uplifting to counter the negative feelings you uncover.
Meditation and mindfulness also help.
I hope that answers your question. If it doesn't or you have more, please post.