Indeed. I see decisions in my playthrough of ME1 I would take back if I could. And in the connected second playthrough of ME2, I lost Garrus, Jack and Mordin on the suicide run. And in my first playthrough of ME2, I elected to save the Collector station for Cerebus. rumor has it that that decision has dire consequences, but keeping with the integrity of the story, I decided to stick with it.
Don't play these like "games", and try to game them, searching for the "best" path. Play this like playing life. Take what you know, what you think you know, make the best decisions you can according to the way you understand the character and roll the dice.