I am terribly sorry if I let you down.

让你失望了，对此我非常抱歉。

youdao

If you put your trust in me, I will not let you down.

你要是信赖我，我就不会让你失望。

《牛津词典》

If anyone here has problems with your shoes, please come to me and I will not let you down.

如果这里有人的鞋子有问题，请来找我，我不会让你失望的。

youdao