Really feed the dog. How to put it mildly?

Sincerely feeding the dog is euphemistically expressed as follows:

1. I used to think that our feelings were deep, but now I find that my sincerity is just a failure.

I am nice to you because I really treat you as a friend, but it seems that I may just be wishful thinking.

I thought my efforts could get your understanding and concern, but in fact my sincerity was just ignored or misunderstood.

I always thought that our relationship was mutual trust and support, but now I feel that my sincerity has just been let down.

I used to treat you sincerely, but now I understand that my sincerity is just a waste.