I agree with the difficult to talk about part. If I'm feeling upset or whatever, I always think that telling people about it would just be a bummer, that other people are better off not knowing and that I don't want to burden them with my emotion, I want to protect them from feeling bad and I know that I can deal with my being upset better than other people can.
I remember once telling my mum while I was in year 12 that I felt really stressed and tired and she overreacted to the whole thing. Her response wasn't what I wanted and I couldn't communicate with her properly, so I then decided that telling people about emotional things in general is a really bad idea. It's very hard to make people understand, even when one is feeling normal sadness, as opposed to clinical depression.
There is a limit to how much we can reach out to others, and I suppose there is a bit of a barrier. Pain is a shared experience in that everybody feels it sometimes, but it's also an intensely private one. I can't properly explain whatever pain I feel to someone else beyond saying that it's 'bad', and this is a pretty poor indicator of the complexity of someone's emotions.
I also relate somewhat to feeling as though nobody would care very much if I were to stop existing. When I was in primary school I was extremely quiet, moreso than now, and I felt ignored. Sometimes I wondered if I was invisible or even if I existed in the first place. I considered that if I just went away, I wouldn't be missed, or at least not terribly. This isn't how I feel now, but it's still a very real feeling and I can easily see how these kinds of thoughts would motivate someone to just go away. I suppose then, I'd encourage people to acknowledge and talk to people who seem lonely and make sure they know that they are important.