If you are a member of a Union, you need to support them just as you expect them to support you.
The Union was trying to protect not only your best interests but also those of the membership, before and after you! If management pulls that crap with you chances are they will do it with others and perhaps they already have.
Hindsight is 20/20, but you should not have withdrawn the grievance. If you think your employer will respect you for withdrawing the grievance, you are wrong.
Sounds like you have a real first class A-hole for a big boss and your union membership is the only chance to level the playing field. I expect, with your union seniority, you're making more than entry level, and the big boss is looking for ways to save money by replacing you or just eliminating your job altogether.
Good luck, from a retired union shop steward!