No matter when you sever the relationship, do it politely. Give as little offense as possible. Most agent-client contracts state that either person can break the relationship by letter or by a thirty- or sixty-day notice. Honor that contract. Be totally professional.
Even if you think the agent is cold-hearted, indifferent, and incompetent, treat her with respect and kindness. Offer the same courtesy you would like extended to you if the situation were reversed.
It sounds strange to say, and ten years ago I would have laughed at such needless advice. Since then, however, I've spoken with several agents and heard their painful tales of dealing with clients who didn't behave professionally.
It's simple to break the contract. Make the parting as painless as possible. A phone call often is enough. If you can't do that, write a brief, businesslike letter to your agent and send it so that the agent must sign for it. (Obviously, you'll keep a copy of the letter and the receipt from the post office.) We like to believe that agents wouldn't try to sue, but we now live in a litigious world, so who knows?
Begin by saying that you wish to dissolve the agent-client relationship. I suggest you provide no reason. To give a reason opens the case for argumentation. The agent may feel the need to defend herself. If you start citing reasons, you may end up writing rude or hurtful things. This is business, not a broken love relationship, and recriminations have no place in business.
It's never wrong to be kind,
even if the relationship between you wasn't good.