I doubt that it's the breasts that are tougher than the past cuz today's factory raised birds don't get out and about much.
Agreeing it's probably the method of cooking.
Possible changes in prepping style for grilling: brine the meat in advance. Breasts only take about 45 minutes to an hour.
Yes to pounding which makes the meat have a level thickness and cooks faster and evenly. 1/4-1/2" thick should do you fine.
Poaching which is lovely for meat being used in salad or pulled chicken type of dishes. You can poach in the microwave or over a fire. Just don't boil the meat cause simmering is your tender friend.
Butterflying the breast. Great when you are in a hurry. Here's a how to:
http://www.goodhousekeeping.com/food-recipes/cooking/tips/g1952/butterfly-chicken-breast/