The difference is between the active and passive voice.
A door closes.
This is active. The door is the subject, and does the action: it closes.
A door is closed.
This is passive. The door is the object of the action: it is closed (by someone or something). In this case, both are describing the same situation.
Likewise, your other two sentences are fine, but they mean different things. Let's start with the more easily understood one:
I get confused sometimes.
This is passive. I is the object of the action: it gets (confused) sometimes.
I confuse sometimes.
This is active, but it is not a good example. I is the subject, and does the action: it confuses (some other person or audience) sometimes. But a transitive verb requires an object; at least an implied object. To make it more understandable:
I don't always explain things fully. I confuse (others) sometimes.