When you start practicing most of your worries will disappear.
It's when you are THINKING about performance that you are afraid.
You need to act and there's no reason why you couldn't successfully play in church in the future.
Since I feel you are the person who wouldn't voluntarily jump into the dark pool at night, another technique is wading. Step by step. Bit by bit.
And no, your 70 years of age shouldn't be a problem either. I have students in their late 80's who are still practicing, trying to improve and taking action.
But it's not possible, if you concentrate on how you are afraid, on how poorly you will play.
Instead, focus on how you can add value to your congregation and help others grow.
[HT to Katharine]
Practice ascending chromatic sequence in F# minor. The chords: I-V43-I6 (see video example below):
"Are there any finger number for the exercise? Or you can use any fingers?"
We generally use interval fingering in each hand, playing the 2nds with 2-3 or 3-4, the 3rds with 1-3 or 2-4, the 4ths with 1-4 or 2-5, the 5ths, 6ths, 7ths, and 8ves with 1-5. In other words, think about each chord in SATB position as having 2 intervals, one between S and A and the other between T and B.
Do you have a question about harmony for Ausra? You can reach her by email.