The word I'm looking for to fill the gap means unsatisfaction and resentment/complain but in a more subtle and formal way.
I vaguely remember a word starting with the alphabets: disc--, which perhaps would suffice here, but can't seem to remember or find. However, I'm not limited to that word only; any suggestions would be welcome.
The perfect word here is 'displeasure'. This clearly states your feelings, whilst inviting the recipient to put things right if they value your friendship or custom (depending on the context). If you use the word 'disgust', they might feel you are so angry it is not worth reasoning with you for the moment, or trying to find a solution. Using the word 'grievance' would suggest that things have gone beyond the stage where friendly discussion might rectify things and that any ongoing correspondence will perhaps involve authorities, lawyers et al.