Maybe because he already came to the realization that Harry was a pseudo Horcrux, he wanted Harry to find this stuff out for himself and make the decision to sacrifice himself? VERY dumb life lesson I agree, but I think Dumbledore tried too hard to piece everything together and didn't want to go off half cocked. He admitted he was dumb for not filling Harry in sooner, and he was weakened by foolishly wearing the ring.
Before he put on the ring was his window of action, but like I said, he was still working out the pieces as well. He could have went guns blazing, but with a Horcrux still out there, Voldemort wouldn't die for realz anyway.
Yeah, the woulda's and shoulda's will getcha everytime. I think the same thing when listening to the books though. JUST DO THIS! or JUST DO THAT! They end the exact same way though.....