Yesterday my uncle, who is a travel agent, offered me a free ticket to Hawaii, but I couldn't take it since I'm having an exam this week which makes me stay.
Now I'm staying home and [the verb/verb phrase].
My first thought was envy and regret but envy is a transitive verb and you envy someone but there is no one else involved here. Regret on the other doesn't fit here, I think, since you had no choice and if you could get back in time, you would choose to stay.
So I'm looking for a word which basically means envy hypothetical yourself if you could've done something but you couldn't so now you feel sad.
Edit: I mean something like daydreaming but it does not bear any pleasant feeling since you missed the opportunity that could bring or could have brought the pleasant outcomes. On the other hand, you had no choice so you can't really regret it because regret implies that you did something wrong or you wish you had done something differently, but here in my context I couldn't really do anytging otherwise. Then completely aware of the fact that you couldn't take the chance anyway, you think of what would've happened if you could and feel kind of sad.