In your situation, to be honest, if you take that kind of English class, you may not be able to keep up with classes about your age. Going to a small class may be unbearable.
Hiring a tutor can give one-on-one guidance according to your situation, and the progress will definitely be great. But the cost is relatively high.
Self-study depends mainly on perseverance. If you can stick to it, this road is actually the best choice, because "you know which shoes squeeze your feet the most". You can start with some simple books. I suggest you learn new concept English.
You should make it simple from the beginning, and don't always find it difficult. English is actually very simple and can only be spoken.
Finally, I wish you success.