Re: ....Faith and Trust....
Sara, if you think you have what it takes to join and be regular at the gym (or whatever else it is that you want to do), then do so. This should provide you with an opportunity to prove others wrong.
People are skeptical of me for a whole bunch of things I do. The only way you can change their negative thinking is by proving them wrong.
Many a times you won't have anyone supporting you. Have faith in Allah (SWT), make plenty of duaa and do whatever it is that you have to. Hopefully, you'll come out successful. Often it is also good not to go around telling others about your problems. Not only would you get 0% support in return, but people would often use it against you.