Taxes

So my question is about taxes, namely how to sell long term vs short term. A little background I have crypto on robinhood mainly and I have transferred a majority of it to a cold storage wallet, what my question is when it comes to taxes how do I differentiate the long term vs short term, for example say I had 1 btc on robinhood that I bought last year, transferred it to cold storage, then I bought another .5 btc and left it in robinhood, now I want to sell the 1 btc by transferring out of my wallet back to robinhood( or wherever)and not sell the .5 btc that I still have on the exchange how do I do that when it comes to taxes.. I admit I didn't record my buys and sells.. am I screwed? In this example say to get all the btc it was over 20+ transactions, can you help? I know now that I need to create a log recording my buys and sells