Never thought of being atheist as a choice. No more than not believing in santa is a choice.

First off, you need to stop keeping track. If you're unsure of your decision then you will let it haunt you and you'll keep track and you'll have doubts etcetera. Although a lot of people who are strong believers seem like they have a disease, you didn't have a disease.
You just made a rational choice . Try to think about how different you live your life now than you did then. I think you'll find there is no difference. You're just not relying on artificial bullshit anymore.

Congrats

It'll all go eventually. De-programming takes time. You have to get into all the little corners of your head and make sure you got it all.

Yoga meditation and psychedelics have been great tools for me in discovering what my underlying assumptions about myself and the world are. I’m sure it depends on how thoroughly and destructively brainwashed you were and from what age but after 10-14 years of seeking I still think I’ve got some damage left to undo yet. I’ve relieved myself of a lot of it but there’s always a little more to work on. Re-educating yourself emotionally and intellectually takes a lot of work and I’m not sure when I’ll ever be satisfied that I’ve gotten myself to the point I could have been if I had just been taught to love myself, but it does get a lot better with introspection and time.

It takes time. For years after I became atheist I couldn’t read books or watch movies about Satan/Devil/whatever because they still scared me. Now, I love reading the mythology of it. Though, I’m still likely to swear or shout “oh god” during sex.

I wish certain parts of my taught morality weren't so ingrained - however - those parts I disagreed with? I don't follow them any longer. And that's a relief. (Not that I ever did follow the stuff I disagreed with - I was a lousy Catholic).

At least as a person who has "become atheist" you understand why so many are still fixated on religion. I think in some ways it can make us both more understanding and less so.
I try to remember that indoctrination often was not anyones choice.

And it can take time to come out of it. Being patient with yourself is a given.

Lingering effects are normal...think how long you have lived your life as a religious person. Consider the famous quote from Karl Marx about religion: " "Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people". Many people feel they need it without questioning the control it has over them. Freedom from religion is liberating...at least that is the way I felt about it when I became an an atheist in my mid 20's.

You worry too much. You are free. Enjoy the freedom.

How does one decide to be an atheist? I've never believed, but I don't think there was ever a conscious decision that I made to not believe, and I am certain I couldn't decide to become a christian or a buddhist on a whim.

It's your past, and it shaped who you are. Don't try to forget it (you can't); the trick is to figure out how to recognize this kind of shit from now on, before you go for it again. Good luck, brother!

Over the years I have known several dis-enfranchised Roman Catholics who still feel guilt about disregard church practice. Just ride it and let your intellect inform you instead of your history.

How does one decide to be an A-theist?

Either way you want to handle it, it's all in your head

For me life is a journey, enjoy the scenery

It's a trial to be sure but stick with it

Eventually, I merged with my own spirit. I carry things that I learned from my religion, that promotes life...but, I do that with everything I experience. The newness will wear off and you will still be your self, only you will have evolved! Best of luck on your new journey!