Due to your conditions, your options are really limited. Obviously meeting people online to talk poses some issues (being that you’re a minor). I suggest to look into shadowing. It’s the act of listening and repeating what you hear in the same way you hear it. This will improve your listening skills and give you practice saying things. (Part of the reason why you may be tongue-tied is because your brain can overloaded with thinking about what words you need to say, how to say it, etc. This process can cause one to lose their train of thought). After several months of doing that, it will be easier to say certain things because you’ve had plenty of practice which frees up resources to focus on what you want to say.
Another thing you can do is narrate what you’re doing, thinking, and/or things you want to say in Japanese. Obviously without a native speaker this won’t prevent you from reinforcing unnatural or incorrect utterances, but knowing a concept and being able to produce it in speech utilize different parts of the brain.
I suggest you also look through the resources on the Ultimate Resource thread for additional ideas. I wish you the best!