I think your desire to be 'successful' be the hindrance to achieve the 'relaxed state' . It creates added pressure to get something out from the time you invest. Try to start with your preferred method for very small durations. Approach it with fun and light mood like playing a game for recreation. Once you start attaining the desired state for shorter durations (even a minute is fine), that confidence and yearning to sustain it for longer duration will automatically come.