How do you come up with something "original"?

[NobodyNothing]

The short answer is:  you don't.  Sorry.  There's nothing new under the sun, as some dead bloke once said.

What you can come up with is a new way to say something old, because although you're not especially unique either (bad luck, emos, people actually do understand you... although I'm not guaranteeing they care), you do have a set of experiences and abilities that is at least a little bit different to the next person's.  

This is something that's hard to grasp when you're young and not well read enough.  Through all my years I've been astounded, then increasingly made aware of the fact that just about everything I thought has been thought before.  And in depth.  Name a topic, any topic.  You're not the first, and certainly not the best.  

Sometimes I think that, besides of what you speak of about the potential "uniqueness" of your experiences, that it's those who can synthesize human experience the best at this time, and do it well, are the poets who will resonate the most with us.  I think synthesis trumps "perceived" uniqueness of insight, feeling, style, etc...

I don't know...I just find myself reading the best "synthesizers" of any particular domain/discipline of thought and endeavor of late more than anything else.

Having said that, "The Art Of Poetry" by Paul Valery was a good read to me about things like this.