Switching letters around in string

So, I am having a problem that seems that there is an obvious fix, I can’t help but look very deep into my own work to not be able to spot the problem on the surface, so I have turned to others, you, for help. I’ll try to best explain this code:

This isn't all my code, just everything that has to do with the subject. Whenever something is at a certain angle, the according letter is put into a temporary holding string (currentLetterSelected) to then once the angle is equal to 0, the temporary string is then equal to the main output string. The temporary string can only hold one letter at a time while you are selecting the letter you desired, but, if you make a mistake and accidentally selected another letter, you cannot go back and its quite annoying. I believe I accomplished the “switch letter” with the deSelectLetter(), but the main problem is that once one letter is selected, isCenter = false so you can only confirm that letter to the main string.

 public void deSelectLetter()
     {
         //currentLetterSelected = currentLetterSelected.Substring(0, currentLetterSelected.Length - 1);
         print("LetterOutput: " + letterOutput.Length);
         print("currentSelected: " + currentLetterSelected.Length); 
 
         if (currentLetterSelected.Length > letterOutput.Length)
         {
             //currentLetterSelected = currentLetterSelected.Substring(0, currentLetterSelected.Length - 1);
             currentLetterSelected = currentLetterSelected.Remove(currentLetterSelected.Length - 1); 
             print("Replace"); 
         }
     }
     public void letterSelect()
     {
         //Switch to Switch funtion, use if statements for proof of concept 
 
 
         if (axis.angle <= 27.6923077f && axis.angle > 0f && isCenter) 
         {
             print("a"); 
             deSelectLetter(); 
             currentLetterSelected += "a"; 
             //currentLetterSelected = currentLetterSelected.Substring(0, letterOutput.Length - 1); 
             isCenter = false; 
             
         }        
 
         if (axis.angle <=55.3846154f && axis.angle > 27.6923077f && isCenter)
         {
             currentLetterSelected += "b"; 
             //currentLetterSelected = currentLetterSelected.Substring(0, letterOutput.Length - 1); 
             isCenter = false; 
         }
 
         //First check to see if PrimaryThumb is centered 
         if (axis.angle == 0f) 
         {
             isCenter = true; 
         }
     }
 
 
 public void Update()
 {
 letterSelect(); 
 if (isCenter == true)
         {
             //currentLetterSelected += letterOutput;
             if (letterOutput != currentLetterSelected)
             {
                 //print("Astra: " + astra);
                 letterOutput = currentLetterSelected;
             }
         }
 }

I was hoping you could see a potential fix, and I am willing to answer any questions you have, and thank you.