Well, focusing on the 'all my fault' part isn't the greatest answer for being happy, because it's judging/condemning yourself ..... saying to yourself, self you could be happy but you aren't and it's all your fault, and it's sort of a circular thing where the self-judgment "all my fault' thing keeps you in a place of not happiness or not contentment or not satisfied.
There's difference between that self-judgment/condemnation thing and taking responsibility for choices/accepting consequences.

But I do understand. I don't blame anyone else (even my folks for nature/nurture) because I consider myself capable of knowing, judging, choosing ... I know many times I have chosen the thought processes that keep me in or send me further into emotional tailspins, thoughts that perpetuate the disquiet within. It's just that it's such a damned hard struggle to re-focus those thoughts .... sometimes it seems much too hard, not fair that it's harder for me than others (or so it seems to me!) or I am weary, so I just go down the more well-worn rut, and then I am disgusted I did it, and it's challenging to get out that cycle of 'negative thought, then negative thought about allowing the negative thought that keeps perpetuating the negative emotion' ... blah blah blah.
But getting into a cycle of blame doesn't get one out of the cycle of negative thinking/depression state.

Some self-compassion that says, "OK, I made some choices here that didn't help me very much, but now I have the power/ability to make some choices to help me change this; I can do it!", will serve better.

And then the question is, what is happiness? What is the pursuit of happiness? What does happiness mean to me; how do I define it? How would I know or believe I am happy? Does having to fight off depression or deal with intense emotions at times mean I really couldn't have contentment or peace or joy in things in my life?

I've always wondered if I was happy, thought I wasn't happy, couldn't be happy. I wouldn't call myself a happy person necessarily, because I'm not a sunny, bubbly type person .... but I had a very limited definition of what happiness could be, thinking I HAD to be sunny, bubbly, optimistic, positive, and it was 'all my fault' I didn't have that nature. Well, I've got the nature I've got and that's what I have to work with.
So I've changed my personal definition of happiness .... and find that acceptance of what it is, even acceptance of my struggle with depressive tendencies/intensity of emotion, and mindful joy, is what happiness is for me.

Anyway, I AM sorry it's a struggle for you, and it's a struggle I relate to and have compassion for you in.