KNOW YOUR LEVERAGE
In any negotiation, you have leverage. Even when you don’t feel it.
Make clear what your strengths are before going into a negotiation. Doing so will make it a lot easier for you to hold your own.
Impostor-syndrome is a real thing, but since you are negotiating in the first place, you have something the meeting planner (or whoever you are talking to) wants. You can also be sure that they want to find the right speaker, find her quickly, and get on with their day.
They are likely to want the whole thing over and done with. Their haste is your part of your leverage. Your upper hand is partly due to a psychological phenomenon called loss aversion. If your deal falls through, the meeting planner will be back to square one. They do not want to lose the time they already invested. Add on top of that meeting planners, are not used to booking speakers, and may feel well out of their comfort zone and need your professionalism.
(That is also why they prefer using bureaus when it is financially viable)
You, in other words, have plenty of leverage.
Make a list of your leverage before every negotiation. Then redo it after the initial talk with the client. If you can get them to talk a lot about their company, the issues they are facing, and the environment they are working in, you can tailor a proposition for them that definitively turns the table to your advantage.
The more they feel you understand them, the more they think they will lose by NOT hiring you. It sounds a little manipulative, but remember that the most natural way to make people feel understood, is by making a sincere effort to understand them. No rocket science in that.
The downside to this is that it takes time. But it may be a worthwhile investment.