For me, I'd start with how it seems you feel your body is under "your" control such that losing flexion = failure on your part. Your head only gets to be in charge of head stuff, not body stuff; that's like deciding your head is in charge of your thoughts or emotions; good luck with that?

Disease processes can have our input but excess weight, for example, is not just the equation of too many calories in and too little exercise out. There's all sorts of other things (shades of gray) that can go wrong between the mouth and the added pounds or missing a trip to the gym.

Notice this company's use of the plural, "therapeutics":

http://www.flexiontherapeutics.com/

There is no all or nothing to them about your knee? And they are totally discounting you or your knee! They are acting like they are in charge of whether you get flexion or not Do they feel they can fail?

Thinking in shades of gray versus all/nothing, success/failure allows for there to be other pieces of the puzzle; you don't just have puzzle apart/puzzle complete? You cannot have a process if you don't have the shades of gray. You are in New York and then you are in San Francisco but there's no Chicago and it might have been Chicago that would help you? I use "school" to help me see it; how when you are in first grade, worrying about graduating from high school makes no sense yet? A black and white picture is merely a silhouette, you don't get many useful features, can't pick out the person in a line up