You're allowed to be disappointed/upset/angry/etc about stuff that happens that you didn't want. You're not allowed to dwell on/obsess over it forever. Well I suppose you can if you want but you don't because you know there's no point ;D Just remember you can be "negative" about it without that in and of itself necessarily being negative.
Does it count if you never had something but you want it desperately but it's so out of reach your best will never be good enough? I had this dream lifestyle that I wanted to achieve five years ago and couldn't make it happen. I still rage about it (mostly internally but it's enough that I get a splitting headache from it) every now and again but mostly don't care anymore and just keep plodding along generally making things up as I go out of sheer spite XD
I actually don't stress out about it being a rather pragmatic guy. Didn't happen, won't happen, move on tends to be the way I act. I regret it, but not to the point of letting that regret spill over into affecting now and the future. It's like my career as a fighter pilot that never happened...Move on. Lol.
It is what it is and there's no point dwelling too much on it as that just means one's now and future may be tainted by that regret.
I make things up as I go along, but add in some goals and plans along the way. I'm not about to complain about my life, I have a good one.