It sounds like your best bet would be to get someone really knowledgeable to watch you sprint. Distance running (ignoring its effect on fast-twitch muscles) often messes up a person's sprint/high-speed mechanics (experience speaks). In particular:
1. Lead with the knee, not the foot. As a matter of physics, your foot *has to* touch the ground a bit in front of your center of gravity; but you should seek to minimize this distance and to have the foot moving backward when it contacts the track, because everything that actually moves you forward happens under and (mostly) *behind* the CofG.
One way to make this easier is to lead with the knee when sprinting. Many distance runners, when they try to get the longer stride of a sprinter, lead with the foot instead--so it lands well in front of the center of gravity (hence is a braking force) and is moving forward upon contact (another braking force).
2. Keep your elbows relaxed. The arms (very slightly) lead the legs, and if your elbows are locked at one angle your knees will not fold up easily--which they have to do, in order to sprint fast.
If the elbow is relaxed, at top speed it will probably open to something like 130 degrees when it's behind your torso, and close to somewhere around 90 degrees when it's in front. (
http://www.statepress.com/wp-content/uploads/2012/04/4.17-Track.jpg
) [The wrist need not be rigid, but the hands should not be flapping at the wrist--paradoxically, this often induces elbow lock.]
3. And be sure the elbows *are* swinging behind your torso! Distance running can get people into the habit of keeping the arms in front (mostly, anyway) of the torso; but remember, *everything that moves you forward happens BEHIND the center of gravity*. Basically, the part of the armswing that you can see is just a recovery stroke; it's the backward swing of the elbow that propels you. (A good mantra for the armswing: "relax-back-relax-back-relax-back.")
Beyond that, some general advice:
1. Work on central nervous system stuff--top speed sprinting, heavy weight lifting, intense drill/form work, plyos, etc.--on only two or (at most) three nonconsecutive days per week. The CNS typically needs 48+ hours to replenish. So don't do flying sprints one day, heavy lifts the next, plyos the third, etc.
2. So far you've gotten a lot of suggestions increasing resistance (and it sounds like you're already implementing most of them). You can also investigate overspeed (towing, sprinting very slightly downhill, etc.), but make sure it's a very slight boost--maybe just a matter of sprinting with the wind at your back--and use it primarily to foster a quicker turnover, not a longer stride.
3. Any kind of work that focuses on developing a movement pattern (shooting free throws, better sprint mechanics) should be done when *fresh*--first thing after your warmup. Do not leave sprint drills--or sprinting itself!--to the end of your workout; do this motor learning/CNS stuff first, and later in the workout you can do basic conditioning (200s at medium speed or whatever).
Good luck. Unless you are ill or something, you should be able to get *faster* at age 21 than you were at 15, so hang in there!--and please bump the thread occasionally to let us know your progress.
