How do I deal with constructive dismissal

Workplace harrassment or hostility is only unlawful if it is due to legally actionable discrimination. In other words, it is due to your race, religion, age (over 40), gender, national origin, etc. (which you did not indicate to be the case). Otherswise, an employer can set the conditions of the workplace much as it sees fit. This means that it can hire/fire, promote/demote, increase/decrease salary, etc. as it deems appropriate. This is known as "at will" employment. Therefore, unless these actions violate the terms of an employment contract or union agreement, they are perfectly permissable. So while not professional they are legal.

You can't do anything. Employment in this country is "employment at will." That means more than just that an employer freely decides who to hire and can equally freely decide to terminate an employee at any time, for any reason. It also means 1) that the employer can change the terms or compensation of employment at any time--change your job, cut your pay, demote you, etc.; and 2) the employer can make the workplace as unpleasant as the employer likes--that is, bullying employees, while unprofessional and morally wrong, is perfectly legal. Since it is legal for your employer to do this--to make your job so unpleasant  you won't want to keep it--there is nothing you can do; there is no legal claim against someone for doing what they legally may do.