Your structured lessons sound good, but you didn't mention how much time you dedicate to study outside of the lessons. Do you have homework? Two conversation lessons a week is really not very much exposure. Between lessons you should be spending many hours each day studying and listening to Thai at your appropriate level. There are many resources available for free on the internet.
Sometimes when studying a language you hit what seems to be a 'glass ceiling' and feel that you aren't advancing any more. You can break through that and move on up, but it takes a big dedication of your time and effort. Best of luck and enjoy the learning process.