I think this last part basically means that she's trying to get over him, but she isn't getting anywhere. The 'on and off' parts could show how it's a constant cycle and her thoughts are always all over the place.

So the first two lines are obvious- she feels extremely alone in the situation. The 'I need to get around this'could mean that she needs to get over her depression/ stop thinking about him.
The second line shows that she wants to tell him how he is making her feel, but at the same time she doesn't want to hurt him- showing she still cares about him.
Now the last line also shows another hint that it could mean self harm. Again, from personal experience I know that the people who knew never really understood why I did it. And no one truly understands unless they have gone through it themselves.

This is so relatable: in my situation me and this guy really liked each other but we weren't going out because he liked someone else too. During this time I couldn't think of anyone else, and I was confused what the heck was going on. This voice in my head was constantly over analyzing what he said to me and what I wish he would say and what I wish I would have said etc.
So I think this part basically means what I just said- she couldn't stop thinking about this guy. I think the first line of the chorus means that she was trying to make out that her sadness wasn't because of him, but she knew it was something else. The second line may mean that she realized that she was actually depressed because of him -not for some other reason that 'she thought she knew'
The last line of the chorus 'I'm unable, come and take me away' could mean two things:
Either suicide (depressing I know)
Or she wants him to come and take her away from the pain so she can stop being confused with her thoughts and so she can escape- I think this option is more likely.

I think that this means that she has this voice inside of her head which is causing her to be depressed and making her self harm. When you are suffering with those things you really can't describe why you feel that way.
The 'what would you do' part- what would he do if he knew how she was feeling what she was doing to herself? From personal experience, I believe that this song is about Avril either breaking up with a guy or having a crush over a guy, and her thoughts are leading to self harm and depression.