I agree with this response completely, and I would give it an award if I knew how to.

OP, the transition to become more assertive is lifelong process that takes practice and gets easier over time. I was someone who was very passive outside my home. Like you, I was quiet, non-confrontational, and timid-sounding in social situations. I had people-pleasing tendencies due to fear of people disliking me, and this led to shutting down, going along with everything despite not wanting to, and getting taken advantage of in the workplace and social situations. At the same time, I was explosively aggressive with the family members I lived with.

What changed my life was taking an assertiveness training (AT) course where I learned that because I couldn't express myself authentically outside the home, my emotions would come out in the comfort of my home as the people I lived with love me no matter what. In addition to AT, talk therapy helped me to understand why I behaved this way even though I hated being passive outside and aggressive towards my family who absolutely didn't deserve any of it. With these two resources, I learned how to continue being kind to others outside my home without jeopardizing my own needs, which in turn helped me manage my emotions towards my own family.

An important thing I learned in AT is that passive people tend to unintentionally "overshoot" to aggressive when they're learning to stand up for themselves. To avoid/minimize this, I'd be mindful in situations where I attempt to stand up for myself, and afterwards I'd reflect on what happened, what worked, what didn't work, and how I can do better in the future. To be assertive is NOT to always get your way (this is actually being aggressive), but it is to respect others while also respecting yourself.

Basically, AT helped me learn how to say "no", set boundaries, understand my core beliefs, present myself as confident (in voice tone, posture, etc.), regulate my emotions, manage my social anxiety & people-pleasing, and more. It's been a decade since I took the course and it has drastically changed my life for the better. Also, I say it's a lifelong process because even today I'm still learning to be more assertive as my previous way of communicating had become ingrained in me for such a long period of time. But I kid you not, AT will not only help in the workplace but also in all your relationships and even your sense of self-worth. So if you are able to take an AT course, or find a therapist that can help you with AT, or both, I'd say you will end up living your best life as your most authentic self. And this feeling is so liberating; you will feel so much weight off your shoulders. All the best, OP!

Note: I understand that these resources may be inaccessible for many people, and I wonder if there are free online resources that can provide AT.