Pronunciation is very difficult to correct if you have been speaking English for a long time. As you know, practice does not make perfect, it makes permanent. That means if you practice incorrectly, it will become permanent. You need to correct this problem before moving forward because having bad pronunciation is one of the most difficult aspects of speaking to someone who does not speak English well.
The pronunciation lessons provided here are American English pronunciation and will provide you with an explanation, instruction, and audio files to help you pronounce sounds correctly. If you don't have access to a native speaker to give you feedback, I encourage you to record and listen to yourself. Although it might be tedious, you'll be surprised at how helpful this actually is.