The 5th finger was one of the very first things I had to learn to train when I began lessons with my first pianist-teacher (A. Schutte, a pupil of J. Lhevinne). I was required to learn to relax the hand so that the 5th finger's metacarpophalangeal (MCP) joint [the knucle that is prominent when you make a fist to punch somebody located between the hand and the fingers] did not sink downward when playing the 5th finger. Then I had to learn to swing the 5th finger from that MCP joint without sinking the MCP joint. (Often I had to use the tip of the index finger of the opposite hand or a pencil eraser under the knuckle to "keep the knuckle up" while learning to move the finger properly). The early efforts of this are most tenuous in the untrained hand; you will feel that you have all the strength of a little song bird. SLOWLY, after repeated small incremental steps, you begin to move it AND maintain the proper shape. Try just holding your hand in a 5-finger position on C major, and just play the 5th finger with a dedicated simple swing stroke, maintaining shape of the hand AND the finger (don't let any finger knuckles buckle backwards). Then you just need A LOT of practice! The exercises by A. Schmitt and then I. Philipp (Exercises for the Independence of the Fingers) are the best for this purpose. Regarding the Schmitt, use a modulating scheme for each exercise, e.g.: C major , C minor [same fingering], A-Flat 7 in first inversion (C, Db, Eb, F, Gb, F, Eb, Db, C, Eb, Gb, Eb, C), up a half-step to Db major, ..., etc. (Practice hands seperate with the one hand doing the finger exercises while the other accompanies with simple triad harmony; then switch.) Do the first part (without the held notes/fingers) before trying the second part with held single notes, held double notes, etc. If you do this with patience and great attention, you will develop tremendously (at least it happens when we are young).