You didn't loose anything. Back then, lets say in 2017, you had 10 bitcoin and now you still have those 10 bitcoin assuming you didn't sell anything. You see, you haven't lost a single dime unless you sell it
Virtually you do not lost the Bitcoin, but the value of Bitcoin in USD, definitely you lost majority of its value.
Stop looking at the fiat values and just look at the amount of coins you have.
It is impossible not to look at it that way. You bought Bitcoin at the expense of your fiat so tell me why should I not look it that way?
It might help you a bit to stay mentally stable
It won't because you cannot deny the fact that you lost more than 50% value of what you spend to buy Bitcoin. Unless you accept the fact that you already lost it. Sugar coating will lead you nowhere just accept the fact and move on.
There is nothing to regret about.
There is, you should have done your own research and acquire enough knowledge before jumping into investment and the best thing, regret and learn from your mistake.
What you have lost investing is lost forever. Just start from the beginning and try not to repeat the same mistakes again.
In Cryptocurrency what you have lost can be recovered as long as you learned from your mistake and be informed of what is happening in the market. Educate yourself and do not let emotion engulf you.