2nd attempt - Trying to find a sequence using only the position of the characters has yielded no meaningful result. It appears that a value for each character is also being used. I think the cipher uses the position of each character in the clear text (from 1 to 123) minus its character value to determine its position in the cipher text. If the calculated position is less than 1, it is wrapped from the beginning to the end.
The following character values are the best that I have found so far. This is not the final answer but the results are closer than any of my previous efforts
Character ? , . + A B C D E F G H I M N O P R S T U X Y
Value 42 41 40 40 40 35 37 38 35 35 31 31 30 24 25 24 25 23 21 18 17 18 13
When the cipher text position is calculated from the clear text position minus the character value, the results are as follows (using "+" to help track the spaces):
My cipher text H A , + E I T G + X T B + T D S T + R E I + A T + E + O T U A N T + C O S M D N A T E O + D N + N U C E Y + H R N U E S . + I H A E T + C P T + R F + + N E ? G I T O Y + I A + C R U Y A + O U D E T + O T U C N H I R U ? E Y + I E H A + P S T + R
OP cipher text H A , + T I T E X G B + + + D T E S T R I + A T + U E + A + O N T C T S O M N D E O T A + D N + U E Y C N + H R U S N . I E T H + A E P T + C R + + F + T N G E Y I ? O I + A + R C A + U O D U T Y E + C O H N T U I ? E U R Y I + H E P S A + T + R
How three spaces come together in the OP's cipher text, is one peculiarity that I do not understand. I'll keep working on this but would be happy if somebody else found the correct solution.