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https://en.wikipedia.org/wiki/Ship_of_Theseus

You've probably hit upon a good metaphor for what's happening.
I believe each time we sleep parts of our personalities are torn down and rebuilt slightly differently.
Whatever the mechanism, you aren't really the same person you were years ago, you're a different person with many of the same memories. The "self" is a useful simplification of reality. At the fundamental level, its not possible to define "you" and "not you" at a moment in time, much less across spans of time.

I went through a period of this when I was younger. I did not find a satisfactory way of dealing with those thoughts, but they did eventually recede.

There's quite a bit of philosophy out there abut this. It might help to read about it. Some physics topics are related, like the Planck scale. If you want to read about what others have thought on the topic, here's a starting point.

So: yes, you're not alone.

I've always thought of that as renewal of the self instead of the self dying.

Your personality is based largely upon your human experience.

As you get older and experience more, you have more things from the world around you to use to orient your thoughts and feelings on the world, and because thoughts and feelings are what the human experience is at its basest level, it will change your personality continuously.

I experienced much the same through and up into my mid twenties. I have found that upon reaching my 30's that it does not happen as much, or at least it takes much more thought and feeling to change my personality.

You too will reach a point where you obtain a certain confidence in who you are and what you actually believe in, and after that, you will not experience the feeling of being a different person every couple of years as much.

My advice to you since you recognize this in yourself is to pay attention to it. If you can realize that it is possible you could be a different person in a couple years, who would you want to be? What would make you happy?

Focus on that, use it.

Because of the nature of time, the universe is in a constant state of becoming something else. Everything is changing all the time. But, because of the Laws of Conservation of Energy and Mass, there is always part of what was before persisting in what is now. For example, a fire burns logs, releasing the kinetic energy as heat, water vapor, carbon dioxide, etc. The heat dissipates because the atmosphere is very large, but it doesn't dissappear, it just gets diluted. The water vapor is released into the atmosphere, and those molecules become moisture in a cloud and turn into rain, continuing in the water cycle. In a metaphorical sense, your past selves have "burned" and "released" what you are now. You may consider your past selves dead, but the molecules that made them continue to exist as your current self, even if those molecules are rearranged or are slightly different (we eat food and excrete waste, so our molecules are regularly being exchanged with other molecules in the environment). Those same molecules were once inside the sun. Before that, those molecules existed at the beginning of the universe. So, in a way, yes we are constantly dying and being transformed, but the stuff that we are made of can never die. We are just constantly changing, along with the universe, because we are part of the universe.

Some folks apparently consider this depressing, but I found it helpful to accept that I'm just a pile of atoms drifting through the universe.

I'm 'alive' in the medical sense, so there's lightnings going between the piles of atoms within my brain and another pile of atoms continues to wobble in the appropriate way to pump a soup of atoms.

But I'm not alive in a sense that inflates meaning into it, which we do a lot:

• the completely religious ideas, like heaven/hell or being reborn (in a sense that isn't just parts of your pile of atoms being reused in other living piles of atoms)
• the widely accepted but undefined 'souls'
• some elevated meaning of 'consciousness' (which does not just mean your pile of atoms has some concept for recognizing piles of atoms as individual objects)

Similarly, the past and the future don't exist. They're concepts we've made up. The whole time traveling brouhaha in science fiction might make one think that they exist more concretely, but that nonsense foots on a missinterpretation of Einstein's theories.

So, there's not a meaning to your past self being alive or not. It really is as simple as it just not existing.

And ultimately, without inflating the meaning of being alive, there's nothing to be sad about either. Because, while it's fancy when piles of atoms do the lightnings and the wobbles, it doesn't matter which concrete atoms are part of that fancy pile.

You can even stop thinking about your pile of atoms and rather consider yourself part of the big pile of atoms which is the Earth or the whole universe. That big pile of atoms is quite immortal.

I highly recommend the book Introduction to Internal Family Systems by Richard Schwartz. It’s helped me a lot, and boils down to the idea that we have “parts,” and that our thoughts and feelings can sometimes be diametrically opposite.

It, along with being able to speak with zero inhibitions to my therapist that makes me feel heard and my thoughts not seem batshit insane, has really brought up a lot of old memories and scared parts of myself. What I thought was anxiety, I’m learning to notice as a fear I’ve had for as long as I can remember, and that fear helped me survive a lot of my early years of trauma.

https://ifs-institute.com

I can guarantee that this book will give you a sense of the answer you’re asking for.

Not quite. To me it rather feels like it's ongoing in the past, continuously, but I can't do anything. It feels like I am an observer of the past me, just watching myself do mistakes over and over. None of the things I can relive. And that extends into the present. I often can't enjoy things even if there is something to enjoy, because I keep thinking "this will be past in the future, and I will never experience it again". Sometimes it even feels like the present is just a memory that I am thinking of in the future.

Personally, I'm glad that fuck-up is dead.

I don't think strangers on the internet will provide you with answers. You will need to put in the work yourself to get an answer.

Questions for you: Are you your environment? Are you your hands? Are you your thoughts? Are you your emotions? Are you your subconscious? Are you your memories?
Are you the experience that you experience? Or are you the part that experiences?
Can you see that part? Does that part changes over time?

I would recommend reading philosophy books and ponder about this stuff.

Depends on how you define death. If death is only physical and biological, then no, you are very much alive.

If death is metaphorical yes, we die and we are reborn every second. We live in constant state of transformation, not crossing the same river twice and all that.

You can go to past, but nobody is there anymore

Alternatively, your past selves are immortal. They can't be harmed. Nothing that didn't happen to them can ever happen to them.