I know exactly how you feel. I think that while true love still does exist, it's been diminished by an incredible surge of selfishness among people in general. Relationships are give and take, yet modern psychology (which I'm not undermining because therapy has helped me a lot recently) often teaches us that it's only important for "me" to be happy. In order to be truly happy, it's important to reach out to others, including those closest to us. While fulfilling our own needs and desires makes us happy for awhile (cheating on someone because we're not getting enough sex, dumping someone in order to move up the ladder at work), it can't compare with the feeling of loving someone in the long run. So I guess the only thing to do is never give up hope that you'll find the person that will never stop loving you more than they love themselves :)
People change, feelings change.