Today I let my parents down on a number of various counts, and I feel really bad about it.

I'm sorry that I'm not that reliable.
I'm sorry that I screwed up your plans.
I'm sorry that I'm behaving so selfishly.
I'm sorry that you can't tell when I really understand what you're saying.
I'm sorry that I can't offer assurances that it won't happen again.

I'm sorry.