IMO, in most cases "prayer" is understood as "talking to God": QunvaD jatlh, but if you want to be more precise, things get dicey. If praying is a request, it could well be tlhob, like qurgh said. A humble supplication might add the honorific suffix –neS. If it's a formal recitation, consider "quoting ritual" revmeH tay. I understand this might be close to what a mantra means, but cannot really say. If you want to emphasize its free form/informality, maybe refer to it as "not-formal speech" SoQ SeQbe'. Otherwise the prayer might be a confession, thanking the Divine being or maybe "adoration", where one is simply describing or explaining the traits of the Divine being.
Regarding "forgive", do you have the smartphone app boQwI'? It gives qeHHa' with a lengthy description of how it might work, if especially if the forgiveness is in fact restoring the recipients honor.
Some of these things just don't exist for Klingons. They have no deities to pray to or worship.
For "forgive", maybe "stop resenting someone/thing": qeH 'e' mev
"Praying" is simply requesting something, so perhaps tlhob would work there.
They are more likely to "respect" vuv someone than show them "reverence".
My advice would be to not focus on the words themselves, but the idea the words are trying to communicate and then translate that idea instead.