Having structure is valuable, but your best-laid plans must be applied with flexibility.
Perfect is the enemy of good, as the old saying goes. Sometimes the equipment you want isn't available. Other times, your elbows (or knees, or shoulders) refuse to cooperate. In truth, these bad days can't be entirely avoided, but they can be managed.
This requires the proactive expectation that things will go wrong no matter how perfect your plan is. You then need to have the willingness to deviate from your plan when circumstances require. These imperfect alternatives aren't nearly as costly as you might imagine, and they're certainly much better than becoming exasperated and bailing out of your workout altogether.
For example, if the bench press station is unavailable and you're forced to do dumbbell benches, floor presses, or even machine presses, at a minimum you'll derive nearly as much benefit, and, in many cases, more benefit.
If you had planned on doing 5 sets of chins, but after the second set your girlfriend texts to tell you she needs you back for a minor household emergency, guess what? Those 2 sets deliver about 80% of the benefit the 5 sets would have, and, if you tend towards overtraining, you'll actually be better off than if you'd done your full session.
Kind of Like Driving
The training process is like driving down a straight road. The act of driving straight, when examined more closely, actually involves constant, small, course corrections.
Many people have the unspoken assumption that a perfect plan somehow compensates for a lack of applied effort. But the reverse is closer to the truth. Massive, consistent, hard effort applied to an "iffy" program will deliver better results than a sketchy work ethic coupled with a perfect program.