As most others already said, the best solution is immersion, ideally by talking with someone. If you can't find someone to speak with that would make it quite a bit harder to improve, but not impossible.

For your situation (being able to understand but unable to express yourself in English) I'd reccommend the stereotypical "think in english". I'd recommend talking with someone (ideally a native speaker, but even a fellow learner is incredibly efficient, followed by writing a diary abd participating in online forums (like you currently already are!).

It's best to have input from someone else who can correct you if you make a grammatical mistake, give general advice on what sounds 'more natural' or 'better' in the language and answer any questions you might have, as well as help you if you're "missing one word" (from personal experience when learning a language it's rarely a 'it's at the tip of my tongue' situation. It's more like I just don't know this one word and I need someone to give it to me).

Another thing I can say about your problem is I also suffer a bit from it. Whenever I try to talk in German I need a few hours to 'warm up' - to just get my brain to switch to German and having a speaker on the other side really helps. I can hear the language to jog my memory and the other side can (and often does) give me input on what they think I might be trying to say. (I'm a B1-level speaker so not even close to fluent but more than enough for doing basic interations within an environment open to helping with language issues).

What I liked to do when I was actively learning German was trying to come up with different ways of saying 'the same thing', seing how they differ in their meaning, potential usecases, complexity, grammar, etc. It helps with vocabulary as well as the "thinking in German" part, as languages differ greatly in the ways they package the same message and taking a comparative approach helps greatly with being able to (casually) converse later. Having a fellow learner or a fluent speaker give you multiple variations of the same thing and analyze the differences for/with you would be a godsend for this approach.