I'm surprised nobody has mention this already, but how about you try and make a scopolamine extract?
That way you would be 100% sure of your dose. You can make your own drops or patches, but I say go for drops better as it is absorbed better and you don't have to go around with a patch. Scopolamine is mainly used for exactly what you need while atropine and other alkaloids are not so much. Be careful even with low doses of that.
I've read about doctors prescribing scopolamine patches and even then you could experience side effects such as delirium even though they make it their job to prevent this from happening.
Make sure you have an accurate scale, as even a tiny error can leave you with unwanted side effects. Scopolamine is an essential drug in the World Health Organization's List of Essential Medicines. So that would be my recommendation if you know what you're doing.
Anyhow, this plant has been used for health problems like yours way before extracts and pills were even a thing and it has been used because it works. It doesn't matter if you get it from Belladonna or some other nightshade, find a plant with a good yield and get working.
And I can not stress this enough, once again be careful! If you experience unwanted side effects, contact a doctor immediately. You've been registered in your medical record that you had this issue, so if you say you used these drops you shouldn't get in any trouble. They'll know that you just tried to take care of your health, and weren't trying to do something stupid.