I am still not entirely sure what a split infinitive is, do you mean "to ever gain"? I'm not sure how to go about writing some stream of consciousness in past tense?
"You ready?" the stewardess asked politely.
I gulped and took a deep breath. "Yes," I whispered, struggling to sound confident.
It was my only chance to gain a place in this orchestra: next year I would be too old. I constantly told myself that there was nothing to worry about as I climbed the stairs, although I didn't really believe it. I realised that if I didn't give it my all, all those months of practising would have gone to waste. The door was finally in sight, I could't turn back now. I muttered some last minute words of encouragement under my breath, before reaching out and grasping the handle.