(((TN)))
I'm so, so sorry you're going through this. Your boss sounds like a class-A jerk who needs to be put in his place.
I don't know what the labor laws are like where you live, but it might be worth your while to look into your rights as an employee. I know where I work, any changes in compensation are given to me in writing, and I must sign off on them. I do not see how it is legal for your boss to suddenly decide to reduce your pay without having given you a good reason or presented the appropriate paperwork.
Usually, changes in compensation are tied to a formal performance review. In this performance review, your immediate supervisor(s) evaluate your performance based on your attendance, quality and quantity of work, metrics, etc. Then they give you a score in each area and award an overall rating. Any shortcomings they find are documented in the review. You are then allowed to read the review, sign it, and give a written response. The review then goes in your permanent file.
If your boss has not based your reduction in pay on a formal performance review, I would request one--and insist on it. He needs to put in writing what it is he thinks you should be doing that you aren't so you have an opportunity to make changes. He shouldn't be able to arbitrarily change your pay without your foreknowledge or proper documentation of his reasons. IT IS VERY LIKELY THAT WHAT HE'S DOING IS ILLEGAL.
I suggest gathering some evidence to fight his claim that you aren't doing your job. When you started the job, you should have received an official job description. If you don't have it now, you should be able to obtain a copy from your HR department. Go through and make a list of any responsibilities or tasks you have taken on in the past couple of years that are outside of that job description. Make a note of what you're still doing. Find ways to quantify the results you're achieving in your position and document everything. Then if Mr. Scumbag tries to claim that you aren't doing your job, you can present him with the evidence. And if that doesn't work, take it to a lawyer. (I'm serious.)
You do not deserve to be treated like this. At all. I don't care what your performance is like. What your boss is doing is unethical and unfair and possibly illegal. Fight it. And if he tries to get you to sign paperwork accepting a lower wage for the same work, don't touch it until you've educated yourself on your rights and gotten an official performance review. I sincerely wish you all the best in this fight.