Try strumming muting all six string with no chord in mind.. Different variations. Just get your wrist loose. Then try strumming with two of the chord you can do best.. Use different strum patterns.. At 15 minutes, your may barely be getting warmed up. Work on getting practice time longer.. Have no song in mind.. Just feel the rhythm... relax and let your mind go where it wants..
That pretty much how I got to doing better