I hear people saying, „Sorry, but I need to tell you,” „Truth be told,” and „Forgive me, but I must say this.”
The problem? There is no sorry-ness, no actual need to be forgiven, and no one wants to hear the truth.
These are just conventions for having duplicity.
You can’t say anything you can imagine by simply specifying, „Forgive me for saying this.” You aren’t asking for forgiveness; you ask for permission not to care.
The alternative? For example, if you need to say, „Sorry for the off-topic, but …”, just say, „I know I’m off-topic, but this is important. Here goes:”.