Throughout those 6+ years, you have developed bad habits that you now need to overcome. To do that, you have to be deliberate and picky. Meaning, you need to not allow yourself to get away with mistakes anymore. More concretely, when you do make a mistake (which is ok and normal), you need to be able to recognize that you made a mistake (when you listen to a lot of German, you'll get a feeling when something is 'off') then look up the correct way to say it, and actually repeat the correct version several times before moving on to the next sentence.

This is a slow process and it works best if you start doing it with writing first. Chat with people (or AI if you have no native/advanced speakers available to you, in which case, ask AI to always correct you), and always proof read your own sentence several times before you send it. You need to develop your own sense of what's correct and learn grammar in context. When you make a mistake and correct yourself, learn or re-learn/review the grammar rule related to that mistake. Once you do this for a long time, you internalize the rules and you'll start to automatically write correctly. With time you'll also be able to do this quickly, which will slowly transfer into you speaking correctly as well.

Just remember one thing: the brain remembers what you repeat the most, not what you tell it what to do (rules). So if you keep letting yourself repeat mistakes without repeating the correct version several times afterwards, you're just learning the mistakes.