A very common problem that therapists deal with apart from anxiety and depression is assertiveness training. Many people who go to therapy are asking a similar question in different ways, “How can I get what I want, when I have a hard time asking for it?”
In order to effectively get what you want, you need to start by figuring out what is it, exactly, that you want. If you want to stand up for yourself, you need to have your goals and your vision well defined. You have to lay out a strategy that is matched to those goals so that you know what it is that you want, and so that you’ll know when you’re not getting it. Otherwise, you’re left with a vague sense of dissatisfaction and resentment.
If you can’t express arguments for your needs then you won’t be able to effectively negotiate. Once you know what you want, and why it is that you want it, then you can make a compelling case for yourself. And, of course you will need the courage to put your needs out there.