As well as the non-fiction, which genres have your written? My understanding of your post is that you've tried more than one - could it be that you've not hit on the one that makes your writing really sing, and might hook in an agent? It's clear from the fact you have got requests that you can write, and you can clearly pitch too, and that's something to take away from this. As you know, it's so rare to hit gold with your first book, and that goes for finding an agent as well as when you are on submission to editors. It's a long, bloody process. There will be people who walk into getting an agent and/or publisher, but for most of us, it's hard.

Chibi is 100% right when she says publishing is a fickle industry that is largely out of your control, and that it is perhaps dangerous territory to link your self-worth into what happens. If I were to give some advice, I would say that putting getting published at the centre of your world is certainly in the short term, possibly in the longer term, going to lead to unhappiness, and that it is better being viewed as a hobby - one that is vitally important and is a big part of who you are, but a hobby nonetheless. Thinking of it that way, and taking some of the mental pressure off yourself, might conversely lead you to write even better than you clearly do at the moment, and might make you enjoy yourself more.

Think about some other writing goals you could have, like entering competitions. Think about maybe joining a writing group, and sharing your work that way. Think of other writing goals that might make you happy as well as getting published.

Also, changing the way you see writing isn't giving up. And taking a break if you need to isn't giving up either. When I felt lowest about my writing, I took a break, a reasonably long one. I found some other things I also cared about and paid more attention to them for a while (one of these was to complete a graphic novel which was in no way intended to be serious or even good, and I enjoyed being creative with the pressure off, so you might like to write for fun, like Chibi suggested). Then when I came back to writing again, the worst of the negative emotions I had were gone. It was the best possible thing I could have done, for clarity as much as sanity.

(Sorry if this is poorly written and rambling - I'm tired but wanted to try to say some constructive).