An agent isn't just for one work. She/He is for your writing career (hopefully). And if your agent is giving you the impression she's just there for one book and if it doesn't sell she's out . . . I think that's not a good agent.
My agent took me on with two books - a very marketable YA (or so we thought) and a very not marketable Adult. We exhausted ALL YA publishers with the YA - and despite a few close calls, we have to put the book aside (for now). The Adult is still making the rounds, getting the kind of rejections we anticipated (great, but too difficult for this current climate). We are still finding options, submitting to smaller houses, even different countries. Meanwhile my agent and I had meetings about what else I could write that WOULD sell. We came up with one awesome idea, but then I had an even better one. I called her, we talked about it, and she came up with the plan to sell it on proposal to a particular editor she had in mind. That book is now coming out Fall 2012. That is the FOURTH book we've been working on together.
Never for a minute did I worry that if that first YA didn't sell she'd drop me. Never for a minute has she given up hope that someday (maybe after the sold YA comes out) that that first YA would find a home. Never for a minute has she made me feel bad for coming at her with suggestion after suggestion for the difficult Adult book.
Rejections are a part of this business. An agent doesn't guarantee a book sale. You have to decide as an author if you are willing to put up with the bad in order to get the good. You will be rejected, maybe for years. I have a friend who couldn't sell her YA for forever, and finally after publishing a totally different YA series, that other YA just came out last week.
I know it's tough. Trust me, I feel deep pain with every rejection. But it's truly just how the cookie crumbles. It's very very very normal to be rejected. Even at the "I have an agent" stage. What you are going through, we have all gone through. And while I know how hard it is to work on another project while feeling so frustrated and just plain sad, that's the lot of an author. The only control we have over our career are the books we write. I wouldn't have a book deal now, if I allowed my sadness at being rejected so much and so often prevent me from writing. I'd just be depressed. So take control, and start work on something that is so totally awesome.
You are capable of it, and quite prolific. I know you can do it. And you'll be so proud of yourself that you did.