The thing with assertiveness in relationships is that timing in communicating your feelings is tricky. If we use nonviolent communication to tackle the challenges of our relationship, then we need to identify our underlying needs. After that, we have to take our time to determine whether we are ready to talk and express those feelings. Our vision can be blinded and compromised by the urgency of expressing an emotion connected to anger.
If I am angry at my partner, and behind the anger, there is a feeling, and behind the feeling, there is a need, I need time to identify all this. And sometimes I want to talk immediately! That is the thing!
It seems that the more you wait to address an issue with someone, the less communication will benefit from it. However, nonviolent communication teaches us that we have to take our time and possibly disengage! How can a passionate Italian like me do that?
My answer to that is writing. The writing helps me calm down:
I write down the exact emotion I felt when a particular situation bothered me
I write down what is the need behind that emotion
If I identify the need behind it, I know exactly which part will depend on me, on my history, rather than blaming the other.
On the other hand, if there is something I'd like to change in the relationship that will entail a compromise, I will have to communicate it to the partner assertively, and in a timely manner.
That is the difficulty in being assertive. Timing is everything, and how you say things is everything. Try to use "I" statements, and try not to blame anyone. Just state what you need.
Example: I value intimacy a lot, and recently, I have had the feeling we haven't had enough moments alone. What if we do something about that?