The opposite of fixed is flexible, so how can one be both?
It's not always easy, but it's definitely necessary. In my opinion, if you can master the craft of what I call "fixed & flexible", you will be golden and life will be breezy.
What do I mean by this? Well... if you want to achieve something and if you have any sort of personal goal, you will need to have a personal plan to get you there. A plan requires structure and structure requires discipline. Despite the fact that discipline is a hard sounding word (atleast to me), this plan needs to be enjoyable. If you aren't enjoying the plan, if you aren't enjoying the day in and day out routine, then I think it's worth asking yourself why? I'm not saying that every step of the way will be fun and fantastic, but if you aren't enjoying any of the process, then the end goal may need to be recalculated.
The thing is, once you've created the structure, life happens and your ability to create flexibility in your plan on both a small and large scale, is the key ingredient to success.
We can't live in a perfect routine forever, and I don't think we would want to anyways. People fall off their targets because the world keeps spinning and days continue to change. People have birthdays, then there is Thanksgiving, then maybe a wedding celebration, then by the time we know it, Santa is sliding down the chimney, babies are born, friend's go through hard times and you need to be there for them, people are sick at work and you need to cover for them, then another baby, maybe a bat mitzvah or perhaps your car breaks down.... its a continuous cycle. If we wait for these events to stop to be able to stick to our "plan" or "get back on track", we probably won't get there, and even worse, we will forget to enjoy and embrace special moments. Therefore, flexibility is essential.
Expect the unexpected, or more like, plan for the non-regular events. Re-arrange your week so that you can accommodate what matters. Try not to miss two "important to you things" in a row, make up for lost time when and where you can. When it's not possible to do it all, accept the fact that you can't be in two places at once and embrace that shift.
If your goals are important enough to you and if you know what you need to be at your best, then prioritize this. What is negotiable? What are your non-negotiables? Ask yourself the question and figure out when to say to no and when to say yes. Sometimes easier said than done, but practice the process and it becomes more natural.
So in summary:
Respect what is important to you.
Make your personal fixed & flexible routine.
Watch it all blossom!