Well, I guess the quickest way to fail is to simply give up. Luckily the opposite is also true... if you don't give up, then you can't fail! Success is guaranteed
Why do you want to lose weight? Think of one good solid reason, hang on to it, and don't let go! That way when anything bad happens or negative thoughts start creeping in, just remind yourself of that reason. I have been told that once you have your "why", the "how" takes care of itself... or at least makes it a whole lot easier
Watch out for your ankles too. I have got really weak ankles as well... I saw a physio and got a few good excercises to do to strengthen them. Maybe try that, or maybe some lower impact excercises like bike riding or swimming??
Keep up the good work