You are male 20 yrs old ,maximum body growth in male takes place between 13 to 21 yrs ,in female it takes place between 13 to 19 yrs,maximum myopic power increases during this period in male & female respectively, now you are going to be 21 yrs old, so your vision & power of eye glasses will get stabilised
Next Steps
Take balanced diet& plenty of green vegetables, there is no direct medicine which directly help in prevention of increase of myopic power.