If it's any help:
I don't think there is any such thing as 'human nature', though I do think there is a lot of complex knowledge in cultures that achieves some of the same practical results.
I don't think the purpose of morality, or life, is to maximize happiness. I suspect maximizing happiness is consistent with the right way of life, if you understand enough, but I don't think it's the best way to look at things, and I think it makes it harder to see the answers.
In general, ideas don't need foundations. "You can't justify that," is not a valid criticism. This includes moral ideas. So if you have a "moral intuition", or think a common sense notion of morality makes sense, but can't justify it perfectly, I don't think that's a problem. It may be a sign of religion, but not a bad one.
The correct way to look at ideas is not to seek justification, but instead to compare them to rival theories. In other words, ask, "Got a better idea?" If there is no rival theory, then criticism is sort of useless. It can help us notice we'd like a better theory and find places to look for new ideas. But without a rival theory to compare with, we can't see which theory seems truer, or which stands up to criticism better, and can't abandon the current theory.
The ideas about foundations and justifications here were best explained in published work by Karl Popper, and also somewhat by David Deutsch (but more to come, he should have a new book out within 2 years). They are not especially popular, but in my judgment they actually make sense, unlike all the rival philosophies.
If you read Popper, be aware that he never wrote much applying his ideas to morality or education. He wrote a lot about science, and about communism and historicism, and also about certain (bad) schools of philosophy, but also explained epistemology in abstract.
Guess that's long enough for now. For what it's worth, I like fielding (non-hostile) questions in these areas.