WORST MISTAKE EVER

So Lee and I are going to Mexico this summer on vacation.  He's spent tons of hours looking into resorts and such.  Well he finally booked one yesterday at 4:15pm for June4th-11th.  This morning I told my mom about the trip, this afternoon she informed me my brothers graduation is June 11th !!  Possibly the 4th, we are hearing conflicting things. 

However, everything was booked - he put a $300 deposit down everything.  Now we need to get everything changed in order for me to attend my brothers graduation.  I called to check on what  needed to be done this afternoon at 3:30pm less than 24 hours later and there was nothing they could do !!!!!!!  In order to switch our flights or reschedule our week we have to pay a $300 fee to change the flights or loose our $300 fee and start over with the planning.  In which case the price will probably increase.

In other words im SOL.  I told Lee I would pay the $300 out of my own pocket because it is my own fault.  Im so conflicted my heart is broken in two.  I know its not the end of the world but im disappointed in myself.  Lee is pissed.  Im stressing.  I just dont know what to do....and the bad thing is it honestly to God slipped my mind that my brother had a gradution or was going to graduate.   And to be honest he has missed some much school because of snow that  they could possibly push his graduation back. 

So if we rebook or cancel the trips I have to come up with $300 and then some for the trip.  I know in the long run this will be nothing and i'll laugh but now in the moment its AWFUL.  I feel obligated to go to my brothers graduation and I know if I dont that I will be letting my family down and they will always resent me for it.  I just wish I had answers.....