I guess the confusion for me still lies in whether those plans that “will definitely be carried out” necessarily involve details like marriage. You said, “If God's purpose is to sum up all things in Christ, like the Scriptures say, then whether or not I get married isn't all that important.” Yeah, and so it’s pretty easy to say, “Well, God doesn’t care either way, therefore whether or not I end up as a wife and mother is totally due to things like how attractive I am, how mature these boys are, the decisions I make in relationships, etc., etc.” Of course, this is a very anxiety-inducing way to think of it! It kind of makes things all my fault if they don’t go “right,” rather than being according to Gods Will. So, naturally, I don’t like that idea and it doesn’t sit right with me when I consider God’s goodness and love for us.
It's a tough balance, for sure, because in one sense, it doesn't matter if you get married or not in the grand scheme, because God's purposes will be accomplished either way. But God is ultimately in control, not just of the overarching themes of history and your life and mine (although you're right, that is extremely helpful for perspective to keep in mind), but of the details of our lives as well. Look at Psalm 139, for example. God knit you together. He sees your going out and your coming in. He knows your thoughts. He "hems you in" from all around (a pretty amazing thought!). He wrote down all your days before you ever existed. That says to me that he is intimately involved (although mysteriously for sure) with the daily stuff of our lives.
The problem with, "God doesn't care either way. He's powerful enough to work with whichever way I go" is that it quickly becomes two things: one of which you've noted, which is Oh Crap, it's all up to me. But the other is, well, then, I can do whatever I want because God can work with whatever I give him. That's true in a way, but we're still accountable to him for our decisions. I'm not saying you're going that direction, but historically, that's where Christians have gone -- either to over-emphasizing our own responsibility in living a holy life or under-emphasizing our accountability to God to live rightly.
So... it's a tension, and something I get pulled back and forth on in a big way. Yes, God can work in spite of my deficiencies, but yes, I also have a responsibility to participate in my sanctification and move toward holiness, because God is at work within me. I think it's helpful to frame it like that. God's work is at both ends, making my work possible. Philippians 2:12-13 is basically my favorite verse because of this: "work out your salvation with fear and trembling, because it is God who works in you, to will and work according to his good pleasure." God is at work, so I can work (and live and pray and do my daily stuff) both with confidence in and awe of God, who works everything according to the pleasure of his will.
With the "marriage" issue, the balance (or tension, I guess) is in saying, OK, God cares. He is at work. I have a responsibility to grow in godliness, to strive for those qualities that make up righteous woman, and to seek him -- remembering that I can work because he is already working to sanctify me. Those godly characteristics won't make God bring me a husband (which I've actually heard -- "God won't bring you a husband until you've taken care of X issue in your life" which is incredibly lame). The truth is, if he does bring me a husband, then those qualities will help me become a wise and godly helpmate. If he doesn't right now or for a while or ever (although that's pretty statistically unlikely), those qualities will help me be content and joyful in whatever circumstance I end up in.
I don't think I've ever actually achieved that kind of balance, and I may never. But there it is...