I'm sorry to hear about your difficulties at work. What do you suppose is motivating the unfair treatment? Do you suspect unlawful reasons? If so, you should consult with an employment attorney licensed in your area right away. Otherwise, you should know that an employer can generally treat an employee in any manner it wishes so long as the treatment is not unlawful (or contrary to provisions of an employment or union contract, if applicable).
I hope things improve for you.
The answer to employment law questions is almost always "it depends." You would need to consult in person with an employment lawyer to get a careful evaluation of your situation. However, an employer generally does have a great deal of discretion to manage (or mismanage) its workforce and operations. There are exeptions. An employer discriminate against an employee because of a protected characteristic (e.g., race, color, national origin, age, religion, gender, or disability) or for engaging in protected activity (e.g, raising a safety concern to OSHA). Good luck.
Sometimes the best answer, difficult as it may be, is to make the break and go where we we will be wanted and valued, where the employer will be grateful for our efforts.
My responses to questions on Avvo are never intended as legal advice and must not be relied upon as legal advice. I give legal advice only in the course of an attorney-client relationship. Exchange of information through Avvo's Questions forum does not establish an attorney-client relationship with me. That relationship is established only by individual consultation and execution of a written agreement for legal services.
I agree with the other attorneys. The one thing that caught my attention, however, is the fact that your successor is three pay grades below you. That smacks of age discrimination. How old are you, and how old is he? Difficult to prove, but it might be there. Get a legal consultation.