Starting training your inner monologue to be active most of the time. Describe the things you see, make remarks about them, make jokes, imagine something to say as if someone was there next to you, or if you were trying to capture it for a story. Describe people, places, have entire dialogues in your head, arguments with imaginary antagonists going from logical disagreements to just a pure trade of insults.
Always find the things that seem the most challenging to you and force your mind into that frequency. If you see someone doing a speech and think damn I couldn't do that, then keep trying to do them in your head for the next few weeks until you get it right. Aim for a wide range. Human speech is crystallized into contextual formats: the lecture, the book report, the flirting, the banter, the eulogy, the best man speech, the academic writing, the letter, you will only improve it on a fundamental level if you go through several of them and force yourself to align your creative resources to their constraints.
Your inner monologue has two modes: you can either figure out what to say word-for-word, improvising linearly like a real conversation, or you can use it more conceptually, jumping all over the place, without burdening yourself with word choice and rhetoric too much. You have to oscillate between the two. People who have the second one as standard are much better at coming up with new ideas and new angles on things, but you will have a hard time articulating those thoughts and you might feel like that part of you is "locked" in your head. The first mode forces you to not only figure out how to say things, but to figure out cadence, timing, and tone, and it forces you to adjust length for the situation, so you need to go 50/50 with them.