Be careful with each other, these fragile flames...

Over dinner the other night, a friend and I both admitted that we feel as though we have become negative people. We both said that in high school we were very cheerful, the kind of person who always had a smile on their face (in fact I can remember being asked more than once if I ever stopped smiling), always had a kind word to say, happy. But now, we felt negative, maybe jaded, that we had lost that perma-smile. While we were both able to assure the other that neither of us came off as a negative person by the worlds standards, the fact that we feel this way probably indicates that some sort of change has taken place.
Is this a change that takes place in everyone? Does the weight of life inevitably harden our hearts, even if its just a little bit? I'm in no way saying that my life has been especially hard or that I'm now a cold hearted person. But, like anyone, I have experienced hurt and dissapointment, I have seen things that frustrated and jaded m heart, and I feel like that has changed the way I react to things and people around me. I'm a little quicker to get frustrated, a little quicker to judge, a little quicker to close myself off, a little quicker to take offence. So I wonder, is there any way to heal the damage done? Am I going to little by little be overcome or can I stand up and fight for my innocence, for the light hearted joy and forgiveness that I long for.
Or was I ever that light hearted? Maybe I was just better at hiding it back then and now my tolerance level is lower and I can't quite keep my negative feelings underwraps. Is it better that I'm now being honest about my feelings? Or does it just reveal that I need to change something in my heart?

"I am convinced that tragedy wants to harden us and that our mission is to never let it."

I don't want to feel like this, no it's not meant to be like this...

The events of today have left me wondering, where is the line between being the bigger person and being a doormat? Or is there even a line? Maybe they are the one and the same.
Jesus said, "You have heard that it was said, 'Eye for eye, and tooth for tooth. But I tell you, do not resist an evil person. If anyone slaps you on the right cheek, turn to them the other cheek also. And if anyone wants to sue you and take your shirt, hand over your coat as well. If anyone forces you to go one mile, go with them two miles." (Matthew 5:38-41)
This passage flies in the face of the idea of standing up for yourself and avoiding being trampled on by other people in your life. However, these sentiments only seem to apply to certain types of relationships. If I were in a dating relationship with someone who was causing me hurt (I'm not talking physical abuse, just not treating me the way I think I deserve to be treated) no one would expect me to stay and try and fix the relationship. However, if I was having the same problem with a friend, walking away would be frowned upon.
When do you stand up and fight and when do you turn the other cheek?
Or should we always turn the other cheek?
How do I stay true to my desire not to cause others pain and sadness without putting myself in a situation that hurts?