Conveniently, happier people tend to achieve more, so you actually don't need to sacrifice happiness to do more good. You might need to sacrifice certain kinds of happiness (e.g. buying expensive cars), but the research on positive psychology tends to show that that kind of thing doesn't contribute much to happiness anyway.
That's not to say that the choices which would lead to the happiest possible you are the exact same ones which would lead to achieving the most good, but there's a lot of overlap, and anyone aiming to do massive amounts of good should not neglect their own happiness.
Relevant LW posts on positive psychology:
May I suggest, the resource you're referring to might be more like stress tolerance, rather than happiness?
If people could save the drowning child without even getting their shoes wet, and no one had already done so, then people are even more monstrous than I thought.
Is "make the world better" not an item in your "my happiness" set? If you are indifferent or antagonistic to the world getting better, this is probably the case. Are "you" not an item in your "the world" set? If you were not born on this Earth, this is probably the case.
I'm not sure where you're going. Other things being equal, making the world better is a good thing and makes me happier. But there are many cases where the world getting better is in opposition to me being happy: if I donate $X to the most effective charity that does a lot of good, but also means I have $X left to spend on myself. The good done by the donation would make me happier, but spending it on myself would have substantially more of that effect.
When the limiting resource is money it's quite clear that we should prioritize the uses where it goes the farthest. If there are three organizations that can distribute antimalarial nets for $5/each, $50/each, and $500/each we should just give to the first one. Similarly, if I have $5 I could use it to have my electricity be generated by wind or I could use it to fund distribution of an additional antimalarial net. I can't spend that $5 on both, so I have to choose, and I choose based on which I think will do more good with the money.
When the limiting resource is happiness, however, prioritization comes less naturally. I could stop taking warm showers, take the bus instead of driving, spend less to donate more, go vegan, donate a kidney, not run fans in summer, or do any of a very large number of things to make the world better at some cost to me. The more I do, the better, but the less happy I am. If I chose options without looking at how they trade off my happiness against benefit to others it would be like choosing what clothes to buy based on how much I would enjoy wearing them and not considering how much they cost.
I also posted this on my blog