I understand a lot more than what I can say.

This will always be the case. The same is true for native speakers. Fortunately this is not only inevitable, it's also a good thing! We only need to know one way to express one idea, but speakers may say the same thing in many different ways, and we need to be able to understand them. If you continue with your input, both your passive and your active vocabulary will keep growing until you get to the point where your active vocabulary is big enough for you to be able to communicate in every situation you need it for. Your goal is not to “activate” your passive vocabulary, but to keep growing both passive and active vocabularies. If you think you need to "activate" your vocabulary to be able to produce words that you can already understand, check out the answer to But you need to practice speaking to be able to speak! You should speak as soon as you can.