DeAnn, part of the problem, the "lifting & splintering" is inherent with the use of treated lumber for decking, especially "2 X" treated material. Treated is great for deck framing, but lay it flat with one side exposed to the sun & weather, and ongoing maintenance will be required to keep the surface "foot friendly."
If it's currently nailed down, you'll need to replace the nails with deck screws, or add screws, to keep the stuff tied to the joists & not popping up after a few months. Cupping & splintering within individual boards does ease up with age, but until then sanding & smoothing the surface may be a yearly job. I'm not positive, but staining may help deter some of the problem, but you shouldn't expect it to prevent the need for more care in the future.
If you have boards which are "cupping" where they hold water, I'd pull them up and flip them over, re-attaching them where the original deck surface is now the bottom side of the board. Too often when building a deck the natural grain of the wood gets ignored, and boards which should have been turned over from the start will begin to look like gutters in a bowling alley.
DeAnn, I'd temper my costs & labor thrown at this deck in accordance with how long it might be before I might look into replacing the decking with a better-suited surface material. Best of luck