Owing to various reasons, I ended up having to sing at sung Mass in Hobart this Sunday by myself (there weren't even the usual copies of the Ordinary chants available for the congregation, so they could only join in from memory), which was extremely stressful!
The Propers being beyond me in their full Gregorian splendour, instead I sang all of them to the psalm tone for mode one introit psalm-verses (plus an adaptation of the opening Kyrie of the Messe Royale for the Alleluia itself).
I had naturally assumed that there would be other singers, so I had decided for their sake and that of the congregation to use the usual Sunday setting of Mass XI (Orbis factor), together with Credo I – despite not actually having sung them for quite some time (since usually I am serving at Missa cantata rather than singing)…
I somehow got through the Kyrie and Gloria (with some falterings and false notes all too obvious to myself), and then in due course Father intoned Credo III – so I had to make a very quick page turn to find it. The Sanctus was alright, sort of, but when it came time for the Agnus Dei, I looked at the music and faced an absolute blank in my memory for the opening notes, so I again hastily turned the pages of my old Liber Usualis, this time to Mass XVIII, and sang its rather simpler setting of the same text.
As for the Offertory and Communion, I sang the hymn Jesu dulcis memoria during the former and Adoro te devote during the latter, which filled up most of the time (as well as being appropriate to those moments, I trust). At Father's suggestion, rather than the simple Salve Regina, I sang "Hail Queen of heaven" as the recessional hymn, which proved acceptable.
I realised very quickly that I had taken on a task that was too hard for me by myself, but short of running away during the sermon (which I seriously considered doing, as I'd already been to a vigil Mass), I had no choice but to persevere. It was very stressful and I don't ever want to have to do it again, at least not without knowing in advance that I will be alone – one attempt at "Missa Unicus et pauper sum ego, for one voice" is quite enough.