In my case, I got sick of being pushed around so I've turned into a "speak my mind" sort of person and tend not to care if it alienates someone because "if they really care then they'll care about my needs too". I try my best to be respectful, but when people are being buttheads I don't mind being more coarse with those folk.

I do have a much harder time being assertive IRL though. For example, I work in home health care currently and the company I'm with now has been absolutely horrible. I'm losing a client partially because of their lack of communication, but also because of them not firing one of the girls who by definition verbally abused the client and the client's daughter is rightfully upset about her still being here, so is switching companies. Lately I have been very short with this person. It's very hard for me not to just tell her exactly what I think of her, but I know it would be crossing a line if I did. She and another also constantly make mistakes, both big and small, and I've only confronted them once each on them and one stopped but this chick who is making us lose the client hasn't. Thing is, most companies take care of complaints caregivers have about other caregivers, and especially in the case of abuse. This company couldn't give a flying eff, so if I see a problem that needs solved I have to do the confronting myself and that has been very hard for me.

I've also had problems in relationships (both friends and more than so) being too assertive or not assertive enough.