When you say you lack “strength” to speak up, what I hear is you lack confidence. One way to increase confidence is to develop skills. As your ability to do a behavior increases, your confidence to do it also increases.
The other way to increase confidence is to address your thinking. Are there beliefs or attitudes contributing to your lack of confidence? Unless your thinking changes, your behavior is unlikely to shift by much, whether you practice new behaviors or not.
So, let’s start there.