The Cowboy Contract by Janice Whiteaker

Ten years ago, Leland made a drunken deal with his best friend. Agreed if they were both single at thirty-five, they’d marry each other.
He’s counted down every damn day since.
But when her birthday comes and goes with neither of them saying a thing about that pinky promise they made so many years ago, he’s faced with a decision.
Confess everything—tell Paige he’s loved her since they were kids and was just too young and stupid to do anything about it. Or keep his fool mouth shut.
If Paige doesn’t love him back, he could lose her completely. But watching her find someone that’s not him would be the worst torture he could imagine.
The risk versus reward is enough to stall him in his tracks.
Proving he’s older, but still might be just as stupid.