I had a similar issue as the OP with the first couple of briskets I did. Thought I overcooked it as my previous experience with meat was if it was dry and tough then it was overcooked. Just the opposite.
I went off of other information on the internet that went off of internal temp rather than probing the meat for the butter feel. After digging through this site and asking questions the "probing like butter" came up again and again regarding the bigger cuts of meat. Then I had doubts if I would know this probe like butter feeling once I achieved it, but the Popdaddy thread linked by BBQ Bandit would have reduced my learning curve had I found it earlier. In any case, once you finally experience it first hand it will make good meat alot more consistent and your internal temp will be more of start to probe reminder. Until you're like one of the Jedis here that are able to ditch their meat thermometers all together.