I find this topic ironic because I'm currently trying to rewrite an old fic of mine, and I have four different versions of the opening scene because I cannot decide which is best to go with in the long run, lol. I know the exact way that I would like the story to be structured, but the best way to introduce that structure is not the best way to start the story, I'm finding. Aaaaand now I'm overthinking the whole process and cannot get to the next scenes, even though I really want to write them and know what they're going to be. It is a very frustrating process right now.
To answer your question, I think that thinking and planning become too much when you get stuck and cannot see another possibility to "escape" a scene or part of the plot. When you are so set on writing it one specific way and do not try to work around it, you only dig yourself deeper into that rabbit hole. It's always good to try different ways of getting through a part that you are stuck on writing, even if it seems weird and not what you planned. Sometimes it helps you clear your head and make it through.
Since writing the first part of this post (I wrote it on my Mac at a friend's house this morning, but did not get a chance to finish until now, at my own apartment), I tried yet another new way to start my fic and it seems to be working! It tied in nicely with two of the other starts I attempted to write, and I am very happy with these changes. I can actually still include the first way I wanted it to start, only now I'll include that part at the end of the chapter instead of the beginning - I was so set on it being the opening scene, but through trying a new way at starting this fic, I've found a new way that it can work, a way that I hadn't realized was even an option before. It's fun stuff!