Viking, I think it is important to be aware of the difference between being aggressive, passive, and passive-aggressive. Assertiveness is important.
I know it was a joke of sorts, but my psychiatrist wrote me a presciption of "no's". Can be refilled as often as necessary and unlimited supply. Presenting that no, in an assertive manner, was the difficult part to learn.
If you "search" posts on here; you'll find the moderators discuss assertiveness, passive, aggressive and passive-aggressive which might help you break it down into steps.
My opinion, is defining what assertiveness is in your live, and is defining when you need to be assertive. Are there certain times, events or situations where you are never assertive. I obviously had difficulty saying no and recognizing when I'd taken on too much. Still working on the both, the last one in particular. I've learned it's okay to go back and say "I've taken on too much". In identifying times you need to be assertive, think about the past and times you wish you were. Don't tackle alot of "times" all at once. Prioritize them. Which situations are going to make you feel better to be more assertive? The "no" and taking on too much are my first two. I started with two, but had a list, not too long so I wouldn't be overwhelmed, but as I tackle one, I add another; working on two at a time.
I needed to learn to say "no" at appropriate times and thus not take on too much for me considering my current depression/anxiety status. Who did I need to say "no" too. Friends who ask for favors, my son who requests babysitting sometimes (not often) but if I can't handle it; I can't handle it. My work when they call for extra hours. there are others, that gives you a general idea. I guess - now that I'm looking at it; they are tied together but separate sometimes too. Separate because sometimes I'd volunteer for things and I'd not think it through, it would be a spontaneous response. So, I've had to learn to stop, think, respond.
That's my opinion, hope it can help in some way.