Harder as in expecting more of yourself? or more critical? Either way, it might be related to a developing ego. In a good way.
Perhaps you are harder on yourself AFTER losing weight because you have proven that you CAN and your confidence in the ability to change is taking over.
Being hard on yourself is a way of testing the boundaries of what is acceptable to you, now that you have the proven skills and tools to change. Whereas before, perhaps you did not have the tools and confidence and therefore was a bit complacent.
Just remember you can be hard on yourself because you are the boss. Just don't be mean to yourself
