Hey we never said our MC was a good judge of character on whom can keep secrets. To be honest I'm okay with this kind of a flaw.
On the other hand, it's not that he's forgotten about revenge it's more it got put on back burner. I mean he doesn't even know where the hell he is.. it's been way longer than he thought when he first came out and he has no means to actually accomplish revenge yet. Plus, there's the whole "what next" that he's now thinking about too thanks to Toa.