With your current setup, what percentage of time do you spend in the drops?

What has worked for me is to spend increasing time in the drops, using that to condition myself to a flatter position. I set a speed threshold: when I hit 20, I go to the drops, even if I don't need to. Once I was comfortable, I lowered my bars some, and started over. I'm also aware of a truly talented rider who, after wind tunnel testing, wanted to lower his position a little as part of an attempt to set a record of some sort. He used a configurable fitting setup at an LBS, spending time at the shop conditioning himself to gradually lower positions until he was comfortable with the setup, at which time his bike was reconfigured.

In your case, it appears that your lower position needs to be done with the stem? You might consider using an adjustable stem until you get the configuration, and any conditioning you need, dialed in. Then you can put the adjustable on the shelf and buy just the right stem.