You will be a specter, a ghost. You can only observe, not change the past.
Well that addition suddenly made it a lot more boring...
In that case, I'ld go back to the summer of 1990.
I was 10 years old and for the first time, reached the finals of a tennis tournament.
I ended up winning the match after a truly mega epic battle that lasted for almost 4 hours.
By the end of the match, close to 200 people were watching us. All the other finals were long finished already and everybody was waiting for us to finish so that they could start the awards ceremony (there were many tournaments in different age categories at that club and all finals were played on the same day). So the place was packed and everybody was watching us in our epic, painful, exhausting struggle.
I ended up winning in 3 sets with a ridiculous tiebreak in set 3. If memory serves me right, it was 13-11 and the other guy had a dozen matchpoints and I save them all.
He had a couple matchpoints in the second set while leading 5-3, which I ended up winning 7-5 after an epic "remontada".
He had another couple matchpoints in set 3 at 5-4 and then again at 6-5 and then again during the tie-break.
I won the tournament at my very first matchpoint.
It was one of my finest moments in life.
I literally fell down from exhaustion when I hit that winning point at the end. My dad had to carry me from the court. Everbody was cheering. Including the parents of my adversary.
I'm even becoming emotional now, just thinking back about it.
I've carried that match with me all my life as a lesson:
NEVER give up. The (metaphorical) match is only over when the very last point is played. Not a second sooner.
If I could pick a moment in my life to re-live... that's most definitely it.
With the birth of my kids as a close second.