Only got neutral and second!

If you have the time and some mechanical ability, start taking it apart until you find something worn out or broken. It could be something simple. 1 possibility would be a bent shifting fork inside the tranny. A seal set and a good set of tools is all you need to do an estamite.

Im guessing a bend bent shifting fork like WaltCMoto said.
Only down side is you have to pull the motor and split the tranny case, which requires some good mechanical skills. I had some tranny problems with my 2000 RM125 this past fall and was able to pull the motor and all the hardware off but when it came to splitting the case I left that up to Yamaha of Troy. 80% of the bill would be having a place keep your bike for a few days to disassemble it and put it all back together. Carrying in just the case will cut down hudge on overall cost. I think the forks are like 40 bucks? While you have it tore down that far you should most definitly have a new rod installed. I think it was around $120 parts and labor to have one pressed onto my crank, either that or Suzuki sells it as a one piece unit for something like $200-$300. The rod and anything that is even partially screwy in there I would replace since getting that far into the motor isnt something you want to do on a regular basis. If you havent cracked a case before I wouldnt start now, maybe ask to watch but you dont want to mess anything up in here or it could be a $1500 bill.