On priorities I'm hesitant to tell you that priorities come and go, because that probably means that they weren't terribly important in the first place. If at one point you thought sports were the most important thing to pay attention to, and later on you discover that they're only a distraction, it was never a priority in the first place. You treated it like one! That's dangerous, and to say that they come and go only allows for that fact. On the other hand, there are some things that don't have to be priorities. In fact, pretty much everything you do doesn't have to be a priority. Doing your laundry is about as close as it gets to a true obligation. Everything else can be given up with little immediate consequence. Why do you prioritize, as an example, a daily writing practice? It's not that there are immediate returns, but that there are long-term returns. Those are important, and only because you have a long-term vision for your life (somewhat) can you truly say that such a practice is a priority today. These short-term goals have to be inextricably linked with a much larger vision. The big vision should not change. Values don't change either. The tasks you use to fulfill these might, and that is when priorities change—but not values. Not vision.
On planning Don't delude yourself into thinking that you have it all figured out. Don't tire yourself with trying to figure it all out. You can't know every possibility, and if you have the entirety of your life mapped out now, either you have too small a vision or you are omniscient. If you're omniscient, then do your thing. I think you'll be perfectly fine in whatever you do. I suspect that you're not, and you need one thing—adaptability. When new opportunity comes to your doorstep, you need to be able to take it and run. When new, but mediocre opportunity comes to your doorstep, you need to be able to not take it and run. You're wasting your time if you stand alongside your life hoping that nothing goes wrong and nothing takes you off the current path. Something will, so be prepared for it. Wherever you go, take with you an excellent work ethic and the ability to learn quickly. You won't need a plan.
On drifting There's another side of the coin when it comes to planning. You can be so adaptable that you simply drift from occupation to occupation. You'll never believe staying is the right option, because you think that every new opportunity is a better opportunity. You'll never achieve depth in one field. You sure won't enjoy the benefits that come with going deep into one field. This is a major problem and you must caution yourself more against becoming aimless than wastefully trying to aim too precisely. Not only will you be wasting your time by having absolutely no plan, you will actually go backwards. The people that have far too specific a plan, not knowing that they will go wrong, at least have a vision. The best place to be is somewhere in the middle. Have a vision. Make a plan. Adjust accordingly.