Striving for a goal is often more stimulating than actually achieving it but having reached the (initial) goal it's time to set another one. At some level to reach a new goal we have to change so I believe it's more a question of who you become as a by product of achievement.
I've reached similar plateaus in my career but as Churchill once remarked "Keep B*****ing On". I've found that very often that bit of enlightenment one is looking for is just around the corner and it often comes from someone you bump into during the course of exercising the task who, unwittingly, becomes a mentor.
I would endorse some of the other suggestions such as distance learning courses and/or instructing or recreational flying. I would also recommend getting into personal self-development - there's plenty of material out there. I've always been quite a fan of Tony Robbins and his teaching and one of his quotes which comes to mind here is "Success without fulfilment is failure" and also that "Why to is more important than how to".
At some level you have to find a philosophy on life to keep you on track through the boring bits!