So sorry to hear that you are having these difficulties with your medication.

First I would like to ask whether you are having any relief at all from your symptoms, and if so how long after you take your sinemet?

You do not say whether your neuro is a general neurologist or a movement disorder specialist, there is a difference, and PD is notoriously hard to treat - it often takes a while to get the right balance of medication for the individual.

Your doses of sinemet are 8 hours apart. This is a long time inbetween doses and unless you stay up very late the last dose will be relatively close to bed time. It is more usual to take sinemet and for it to be prescribed at four hours between doses. So it could be an idea to bring the doses closer together rather than increasing them, by taking 4 doses instead of three.
The long gap between doses could be giving you a wearing off effect that is making you feel worse.....

You say you are experiencing nausea - there are drugs that help with this. If you are outside of the US a good drug is domperidone. But there are others available in the US.

Sinemet dosing can be tricky, too much and the symptoms can re-emerge, and too little does not work. In addition to this sometimes the body is not good at getting it to the brain for several reasons. One of these is dietary. Protein competes with sinemet so should only be eaten late in the day, and it is often better if you do not eat within an hour of your meds, though different people have different responses to this. Another reason can be that the uptake of sinemet in the brain is not very good. In this case often a drug called Comtan (entacapone) can be added to each dose, or a combination drug called Stalevo can be prescribed. Increasing sinemet is not the only option you have.

You need to discuss these options with your neuro, and in order to help him/her it may be a good idea to keep a sinemet diary so that it is clear when your medications are bringing relief, and when they are not.

Hope this is of some help, and that you are able to find a good balance of relief from your symptoms.

Best wishes
Lindy