how much have you lost?
You don't lose till you sell.
No, that's delusional. You have to value your assets at market.
Let's say you had $1000 and you bought one bitcoin and now it is worth $200. There are two ways to look at it without being delusional:
1. You have $0 and 1 bitcoin, but "you don't lose until you sell".
In this case, you lost $1000 when you bought the bitcoin. According to you, you have a bitcoin that has no value until you sell it. You have $0 plus a bitcoin worth nothing, so you lost $1000. You may gain some money back when you sell the bitcoin, but in the meantime you lost $1000.
2. Mark-to-market: you have $0 and $200 worth of bitcoins.
In this case you effectively have $200 because you can exchange the bitcoin for $200. You lost $800.