Boneless, skinless chicken is sometimes injected with or soaked in brine. It has a funny texture when cooked. You're better off buying bone-in, skin-on and removing it yourself.
However, if you want to go the boneless, skinless route, try cutting it into strips or chunks, skewering it, and cooking it under the broiler for 6-7 minutes on each side. I find that is the only way besides grilling that makes boneless, skinless chicken breast tender.
Good luck :)
We have switched to boneless skinless thighs.
At what temperature are you cooking your chicken? Breasts, if you would like them to be tender and juicy, should be cooked gently until they are 149 degrees F inside. That can mean sous vide (set at 149 degrees, for 1 hour), in a slow-cooker (like this recipe: https://food52.com/recipes...), or in a pot on the stove (like when you poach Hainanese chicken: simmer with aromatics until the meat reaches 149 degrees F). You will have a smaller margin for error with skinless, boneless breasts since they will not have the insulating bone or skin. According to Kenji Lopez-Alt, heat applied to muscle fibers will wring moisture from them: the higher the heat, the more intense the wringing, therefore the drier the meat. (If you've ever noticed the browned meat crust is always tougher than the inside, that's the reason: the surface of the meat that's in direct contact with intense heat gets dried out.)
Dark meat can take a bit more cooking because of the fat, but I still wouldn't heat it past 149 degrees. It will still be juicy at higher temps.
Commercially raised chicken breasts (boneless, skinless, especially) are often injected with a brine in an effort to ensure a juicy outcome. That's surely one reason for that result, because when cooking them comparatively with the boneless breasts I get from my poultry farmer at the Greenmarket, I don't have that reaction.
On the other hand, another reason boneless, skinless chicken breasts come out tough and chewy is overcooking - usually a too high a temperature. Try searing them over medium high, then turning down the heat to medium low and covering the pan partially for the rest of the time.