If you look up compassion and altruism you'll clearly see the differences.
1. an attitude or way of behaving marked by unselfish concern for the welfare of others
2. the belief that acting for the benefit of others is right and good.
sympathy for the suffering of others, often including a desire to help
the ability to enter into, understand, or share somebody else's feelings.
Altruism is the response to compassion/sympathy.
To me it is the most important thing of all to go through life with an attitude of altruism even of you cannot feel comassion in any given situation; it is the only true way to behave in my opinion.
Personally, I try all the time to be altruistic to others and to the world but as a mad man I often times find myself losing control and becoming a slave to my emotions. All you can do is return yourself to the right attitude and keep going. If you reprimand yourself you are still responding to an emotion and you're not being compassionate with yourself.
I think the more compassion we develop for others and ourselves the better we are at having an altruistic attitude. Also, the more we realize that our emotions are transient and to try to respond to them like mosquito bites (scratching at every one) is futile and not good for us and others the more we can walk through life acting in accordance with a compassionate, attruistic mindset.
I am most compassionate when I see through the enevitable cracks in the walls of other peoples personalities and have a glimps of the same weakness that I to struggle with every second of every day. Sometimes I have to check myself though, because I may find myself falling into pity. Pity is not the same as compassion, its just responding to the emotions of another rather than to your own, (or both, perhaps).
I wrote this on the fly so I hope it was coherent.