Try this. Record yourself， then listen to the recording. Can you hear the difference between you and native speakers? What's different? What specific sounds are different? Is the length of each sound different? Once you know this difference, you can begin exercising that sound.
Note: exercise is different from conversation. When you exercise, you work on a specific area, in the same way that athletes and musicians practice on specific movements and musical notes over and over again until it becomes natural.
For quick recordings, your iphone or Android are fine. For recordings on your computer, try Audacity (free).