Only nastiness comes out of stress. Most everyone is "good people" and if given the time and opportunity to think things through are going to end up assertive, instead of nasty.

Don't get confused between what you are experiencing and what someone else says! If you are trying your best, you are not being nasty, no matter what someone else might experience or say. Also don't call yourself names just because you feel pressured or uncomfortable. Someone else pressuring you or calling you names does not mean you have been nasty, it just means they have and/or that they have a problem. Name calling, whether of one's self or others is always nasty.

You're allowed to take your time! If you are confused about something, stop and figure it out. Don't let other people rush you. Assertive is looking out for yourself because, if you think about it, you are all you have. Your stuck with yourself (and nobody else) so you might as well make yourself function well and be comfortable?

You are allowed to want what you want. However, that is not the same as getting what you want. We have to "pay" for what we want in time and effort and even then might not be able to get it. But you can want the moon; your friends can all want to talk to you at once, each one wanting to be first. But they only control half that want, you control the other half and you're allowed to want what you want so you may not have the physical time or energy for them, to help them get what they want and they have to deal with that because it's their want. Just think of the slick salesman who says, "Can I have 10 minutes of your time?" -- you're allowed to say no. Treat demands on you as if they're unsolicited phone calls. Some may be of interest to you but others not. But you get to decide because it's taking your time and energy. Saying no (or yes) is assertive as it is "asserting" what you want, in your space. You can't be assertive in someone else's space because it's not your space! That's being aggressive and you could be guilty of boundary crossing.

Practical example: Two people trying to make an appointment of when to meet. "Is 2:00 on Wednesday good for you. . ." Each person gets to say what is/is not good for them when it's about them. But you don't say, "Well, you have to meet me at 2:00 Wednesday!" to another do you? That's aggressive, not assertive. However, "No, 2:00 is not good for me, how about 3:00 on Friday?" is assertive. You're telling the other person where you "are", you're communicating. That's all being assertive is, communicating with ONE other about where you are in your relationship with that one person so they can negotiate with and work around your needs and desires. You do the same with theirs and end up with a workable solution to whatever the issue is.