As already mentioned, would you buy it if you didn't own it? That will get you around your Loss Aversion Bias. If you're not comfortable with the gambling, well, good, just sell all of it.
Another trick is to make a rule for when you'll sell it. "IF X, I will sell immediately," then stick to it and accept the results.
Diversity on your gamble here would simply involve selling half and indexing it. You don't say how much it went up, but if it went up 50%, you're extremely unlikely to "lose the bet" so to speak if you sell half and put it in an index fund.