It is possible to overcome this fear. All you need to do is to reprogram your thoughts a bit. First, you must remind yourself that you are in better shape throughout the day. For example, it would help if you repeatedly tell yourself that overexercising and dieting beyond limits would diminish marginal benefits. Also, simple practices like talking to friends and family every time you feel an irrational fear might help you. Simple techniques like positive affirmations may help you. Then, seek professional therapy to feel better results.