It happened to me my senior year of high school at the Regional XC meet (the one where you qualify for the state meet). I was number 1 all year and ended up with a DNF because of the fainting. It was single handedly my fault that our team didn't go to States that year (or myself). Because of that I always felt like I had let my team down and because of that I had very little confidence in myself in my racing capabilities.
It took a while to mentally recover from that kind of trauma, but that next track season I really kicked ass, set the school record in the 3200 and qualified for states. Earned a scholarship to a DI school and the rest is history.
My advice is seriously just take it with a grain of salt. You can't totally forget it because you need to know what to change in your training/diet/fluid intake to prevent that from happening again. If you let this hang over your head the rest of the season will go very poorly, I assure you that. Hope this helps.