If you are being strong this means that you are overcoming pain or difficulty in some way. Life can be hard at times, and being strong is one of the traits that make humans overcome these difficulties and end up a better person for the experience. Often it is hard to imagine what you would do if a certain situation occurs, but when difficult times happen you simply forge ahead without letting the problems you face drag you down. That is what being strong is all about.
Stop over thinking, stop worrying, stop complaining. Life is too short for all of that.
Pretending to be happy when you're in pain is just an example of how strong you are as a person.
You were given this life because you are strong enough to live it.