I would say that is true, in general. But not always ...some things happen that are exceeding hard to handle, and sometimes people can't afford therapy (or find a good therapist, but that is another topic). Sometimes therapy doesn't work.
Sometimes people don't have the tools to be happy, or don't know how they can be. I would be devastated if one of my children died. I know that setting up a scholarship fund or whatever, would not help that. I don't know that people can get over that. Whether they can be happy again, I don't know that either.
For me, it depends on my attitude, relationships, community involvement, spirituality, and on my interests and motivations, and on my work being meaningful to me (money doesn't motivate me, making a positive difference in lives does). It also depends on my needs being met, which for me is best described by Maslov's Hierarchy of Needs.
https://en.wikipedia.org/wiki/Maslow's_hierarchy_of_needs
For me, the best example of someone deciding to be happy is my cousin. He is wheelchair-bound with Multiple Sclerosis, his wife died 5 years ago of cancer, his only child has had zero contact with family members since her death, and he can no longer live the very physically active life he had, doing things he loved. He is happy. This doesn't mean he doesn't miss his previous life, but he has accepted and done the best with what he has, and enjoys the support of his community and his close friends.
Fortunately, he is financially secure and can afford the help he needs so he can live at home.
Financial security, in my view, is an element of happiness. If you have to live in a tent on the side of the road, can't afford medical care or food, are addicted to drugs and cannot get treatment, have been traumatized or are actively being abused,but can't afford treatment, etc., I don't see how you can be happy.
I was really lucky when I sought cancer treatment, which would have been denied to me, had I not been able to get on both Medicare and Medicaid. Others have their finances destroyed by cancer care, even when they have insurance.