I bought a bunch of BTC in early 2013. Since then, I’ve bought very small parts of Bitcoin every now and then up until now, for much larger (and increasing) price.

Many of those "original" coins have been lost. While there was a "hard fork gold rush", I gained some more from the ones I already had by claiming "Bitcoin Gold" and "Bitcoin Diamond" and all kinds of such things and then trading them back to real Bitcoin using various non-KYC/AML webpages which allowed me to do this without registering any account. I can’t even remember the name of those services, let alone the exact details of when I did it or how much I got.

I have no idea whether the coins I have left today are all from those original coins, which I bought for next to nothing, or if some of them are part of the ones I bought in later years, at a much steeper price. I have no idea how I’d even figure that out.

With such a messy situation, how can I possibly calculate how much taxes I owe to the government if I were to sell/spend my coins? I seriously don’t understand how I can possibly determine how much "gains" I’ve had considering the messiness of it all.

With this in mind, what am I supposed to do? Assume that all the coins I have left are part of my first batch of coins, even though this will mean that I have to pay taxes on basically the full balance? And even if I go that route, I can’t prove how much I paid or when. (It was Localbitcoins pre-KYC/AML.)

I don’t understand how I’m supposed to figure this out, and the government’s website is (of course) to no help. What would be the most reasonable thing to do? Just "bite the bullet" and pretend that I got all the coins I currently have for free, and thus have to pay taxes on the entire balance?