The purpose of the whole creation is to get to the ultimate goal, in Olam Habaa. Someone who is kind in this world, for the sake of this world, gets rewarded in this world.
The greater the person the loftier his desires. The highest and loftiest desire is to desire Hashem. That is Olam Habaa. You say you’ll shun a small desire of eating your lunch for a greater one, of being able to help someone. There is even a greater thing, to become holy and close to Hashem, eternaly. That is what holy people desire.
I hope you are aware of the famous Mishna about Hevei Ka’avadim Shemishamshim Es Harav Shelo Al Menass Lekabel Pras. We are all taught this Mishna that it is better to worship Hashem just because He said so, and not for the reward. That would suffice to answer your first question. However, as I pointed out earlier, even someone who is doing Mitzvos to collect Olam Habaa, is a great Tzaddik. Tell me if you reached that point that you don’t desire earthly things. A self centered person is someone who makes sure he gets his goodies. A philanthropist who enjoys becoming famous for his accomplishments is not considered self centered. To ask this question is to view Olam Habaa very simplistically, or like a Muslim.
The problem I have is that it sounds like you think it’s better to act for the sake of Earth than for the sake of Hashem.