A lot of fibro patients complain about this. It's the downside of having an 'invisible disability'. If you look fine, then people have a tendency to not be very understanding. Some people I know go so far as to 'look disabled', fake a limp or something - which is just silly! Others I've heard from just don't use to placard in order to not deal with the skepticism - which is also silly! If you need it, you need it.
They should give you the benefit of the doubt. People should look at a 18 year old in a handicapped spot and feel sympathy - what is wrong with an 18y/o that they have to park there? that's terrible. But alas, people are a lot more cynical. Unless you're in a wheelchair they assume you're driving your grandmother's car and taking advantage. Unfortunately, people do that to.
There is a stereotype of 'disabled' which is either elderly or in a wheelchair. It's unfortunately the way it is. All you can do is educate people one at a time - your friends and family and people who take interest. And then just try to ignore people who look at you askance. You know you need to be there.
And just try to keep on top of your fibro. Do your exercises and stretches, eat healthy, manage your stress and manage your energy use. You can get better, a lot of people do. A year ago I couldn't walk at all and now I can handle walking almost 2 miles a day (a small achievement to many, but a big achievement to a fibro sufferer!). Maybe you can get better to a point that you won't need it. But never feel ashamed of parking in those spaces if you need to! Feeling guilty will stress you out and stress will cause you to flare!
Also, you can join a fibro group - either in your area or online (webmd.com has a great one). Talking to other people really helps! They've experienced all the same things and have great advice on how to cope.
So yeah, just ignore ignorant people and get better! Good luck to you!