I will quote myself to make the OP a little more organized.
What do you think human nature is?
Human nature is both the good and bad things we do. It is both the saint and the sinner. It is the sum of our instincts, our learned behavior, our intellect, and our emotions. Of course, I am using broad language here, but essentially, it is everything we like and dislike about our species. It is both yin and yang.
What political philosophy most closely matches it?
I think to some level, every philosophy recognizes a portion of human nature. Fascism responds to our need for order, strength, and leadership. Libertarianism responds to our need for freedom and spirit. Liberalism responds to our caring, desire for growth, and need for community. Conservatism also responds to our need for community, the tribal (national as modernly expressed) instinct, and our need for order and triumph of our group over competing groups.
The problem is that while each philosphy caters to a need within human nature, it also denies other portions of human nature. This is why I try to be moderate. I don't think pure discipline is a recipe for success as the human mind contains a bunch of competing and often contradictory interests, so much society in response. (at least, a free society).
Do you think people in general are capable of change or will people always be what they are from generation to generation? (This is not about individuals changing, but society in general).
While I have my (strong) preference. I cannot wish away the tide. People will always be as they are with the only changes coming from evolutionary pressures. The other possibility (and something I believe will happen at some point) is that there will be a divine reckoning and when we get new bodies, a lot of our instincts contained in our genes will be erased, leaving us with a freer will and more able to make choices than we currently are able to.
However, under our own power, nothing will ever change and the best we can do is to make society a more accurate representation of the struggle inside us, which is the most freedom we could ever hope for in our current state.