Yellowstone cowboy Kevin Costner was fast out of the gate with his new leading lady, Jewel, but insiders claim the whirlwind romance is starting to cool down, RadarOnline.com has learned. Sources claim the singer has pulled on the relationship’s reins and warned things are moving too fast, allegedly causing the actor to lick his wounds. While Costner, 69, and Jewel, 49, are still an item, the romance has allegedly begun to stall. “Kevin and Jewel are still dating but she doesn’t live in L.A. so they’re doing the long-distance thing, which makes it tough — especially because Kevin wants to spen…