My feeling is that being a bully is all about 'them' but how I respond to that behavior is all about me. jmho It's not my fault if somebody else acts badly.
Most of my life I've been fairly assertive and able to keep good boundaries...however there were sometimes when I was completely blindsided by the same kind of behavior you experienced- of being bullied, belittled or dismissed as not being important etc etc.
There are books about this that can help. One I like in particular is called
Speaking the Truth in Love [Paperback] by Kenneth C. Haugk (Author), Ruth Koch (Author) The focus is faith based but I think the information and format would be useful from secular viewpoint as well.
This is a brief review of the book that I found on amazon:
Assertiveness can be defined as "moving toward one's self as you move toward another person." This is to be contrasted with Passiveness (in which a person decides not to reveal his true feelings and ends up experiencing guilt and frustration...thus working against himself) and Aggressiveness (in which a person attacks and thus works against another person). The authors explain why Assertiveness is the best of these three options, hold up Jesus as the ultimate example of Assertiveness, and give practical advice for becoming more Assertive.