It's rather unusual to hear an alto complaining about high top notes in her own part. As a first tenor, I've had to hit the same C on the odd occasion - although I'd probably have a problem with a B these days. I've never really thought of the Mozart Requiem as being that high, but people were smaller on average and voices were probably higher during the time of Mozart. From my point of view as a first tenor, probably the highest tessitura is the Lachrimosa of the Berlioz Requiem.

Probably the fact that you are worrying about your top range is not helping - you need to be relaxed before you hit top notes. Many years ago (when I normally sang baritone) I was asked to sing the title role in a production of "Jack & the Beanstalk". It had been originally written for a top soprano "Principal Boy", but she'd had to back out from climbing beanstalks when she found that she was pregnant. The pitch, even though I was singing it an octave down as a tenor, was rather high for me, so the MD transposed it down a tone or two. After that I had no problems with the notes. It was only at the last night party that the MD told me that he had gradually shifted the pitch back up to the original during the rehearsal weeks and that I'd been getting an effortless top B every night of the production - he claimed that he'd been too lazy to transpose all the orchestra parts !
The point is that I'd managed the notes because I wasn't worrying about how high they were. That was what made me realise that I could sing Tenor.

The same trick might help you, although of course you wouldn't be in the dark about it as I was. Try singing through your part a tone down. This will help to give you a feel for the melody and intervals in your part and allow you to sing it without so much effort or trepidation and you will therefore probably learn it better. Then when you feel more relaxed with the part, try singing it back up to pitch. I even used to know a soprano who would have a couple of glasses of wine an hour or so before she tried to rehearse any new high parts and used to reccommend the practice - having heard her hit fautless high Es on several occasions, I can only say that it seemed to work for her.