AN SMA PLT SUFFERING FROM HYPOXIA FAILS TO SUCCESSFULLY COMPLETE 3 IAP ILS VOR APCHS. GETS FLT ASSIST IN DIVERSION TO ALT ARPT.

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I FILED AN IFR FLT PLAN FROM IDA DIRECT TO BZN WITH ENRTE ALT OF 15000 MEA. AFTER PASSING THROUGH 12000 FT I PUT ON THE OXYGEN MASK AND ACTIVATED THE SYS. HOWEVER; AFTER ABOUT 5-10 MINS OF USE THE REGULATOR MALFUNCTIONED OR THERE WAS SOME TYPE OF RESTRICTION IN THE DELIVERY LINE AS THE OXYGEN SUPPLY STOPPED. I ELECTED TO CONTINUE THE FLT AT 15000 TO BZN BECAUSE THE FORECAST WX CONDITIONS AT IDA CALLED FOR RAPIDLY DETERIORATING CONDITIONS. ENRTE FLT TIME WAS APPROX 2 HRS WITH ESTIMATED 1 1/2 HRS ABOVE 12500 FT. THE ENTIRE FLT WAS IN IMC. UPON REACHING BZN; I FOUND GREAT DIFFICULTY AND CONFUSION IN PERFORMING THE TASKS REQUIRED TO EXECUTE THE ILS AND VOR APCHS. AFTER 3 UNSUCCESSFUL APCHS; ZLC DIVERTED ME TO BTM FOR LNDG IN VFR CONDITIONS. PRIOR TO REACHING BTM A SMALL PVT FIELD BECAME AVAILABLE AND THE FLT WAS TERMINATED WITH A VFR LNDG AT THIS FIELD. (JEFFCO AT WHITEHALL). IN RETROSPECT; ALTHOUGH I TRAIN STUDENTS USING THE BZN APCHS AND KNOW THEM WELL; I AM SURE MY PERFORMANCE; JUDGEMENT AND DECISION MAKING ABILITY WAS DEGRADED BY HYPOXIA AND FATIGUE.