Life is not fair and regret is one of the most painful emotions.
You still have a choice, you really do.
You can wallow is depression and regret or you can start again.
I'm not a therapist or great with words, but I am a guy in his mid 40's that has struggled with anxiety, depression and regret, so I do understand.
You can overcome SA, I have. I won't say that I'm 100% anxiety free, but I can do many things now that I 'could not' do before and I'm happier in my life than I have ever been.
I think exercise, any kind of cardio, is extremely important and Dr. Richards program of course.
You are 40ish, not 90ish, your life is not over, you have many years ahead. Even if you were 90, every day really does matter.
The past is gone, you can do nothing to change it, all the regret in the world does not matter, all it does is ruin your day.
You CAN beat SA and the related depression. Start the SA course again, exercise even when you don't want to, ignore the negative thoughts in your head and do it anyway.
That's all they are, negative thoughts in your head, nothing more.
I wish you luck, don't give up.