How to avoid the constantly changing mindset and have a sharp focus

This is a wonderful question. Also, I can not imagine what it is like to be dealing with what you are. What a daunting situation. Indeed, there could be so many possibilities to explain what is going on for you, that perhaps my response will provide you with more questions than answers. Likewise, what I write here is certainly not jointly-exhaustive as far as explanations go. Indeed, you are welcome to discard any part of it, or all of it if you prefer.

However, here are some possible reasons why someone may notice their interests shifting from week to week:

Some individuals may shift interests from week to week, because they have never practiced developing mastery over something. This may be something that will take lots of encouragement along the way if it is your first endeavor doing so.

For some, there is a fear that comes up amidst what they are working on, and the interest in another area is a way to avoid that fear. Do you notice any fears come up for you as you stay with your work? What about any anxieties? Addressing those may be part of the solution.

You could ask yourself to fill in the blank with whatever comes to your mind first 10-12 times each by starting with these two separate sets of words:

"If I keep this job then _______."

"If I leave this job then _______."

What came up for you?

Some individuals may feel trapped doing any one thing. Perhaps, realizing that constantly shifting from week-to-week may on the one hand feel liberating, but at the same time, could also be its own form of chaos and inhibits one from tapping into professional development in their career.

Others may shift from one thing to another, as a form of self-sabotage. Some people fear success, because success involves change. Others fear success, as it may not line up with one's sense of identity.

Sometimes people get bored with something because it may either feel too intimidating, or alternatively, they realize their psychogenic needs are not being met. In either case, sometimes one may notice that they would be more satisfied in a different role all together.

Yet, those who do notice their psychogenic needs are not met may be able to get those satisfied outside of work on the weekends (eg volunteering at animal shelter, going on hikes with friends, etc.).

Further, sometimes a lack of focus pertains to issues of motivation. As Jon Haidt has pointed out, sometimes, in order to stay motivated one needs to not only appeal to their rational side, but also their emotional side.

Regarding the former, he mentions knowing where to go and how to get there; and being able to break it down into simple steps may be helpful.

Regarding the latter, learning what motivates you most is key (whether it be the carrot or the stick, fear of failure or thriving off success).

For one that is motivated by the carrot, tapping into what one can get from staying in their role can be helpful for them to feel more encouraged to do so. Sometimes an image that represents that good thing can be put on as a desktop background as a visual motivator/reminder.

Likewise, if you are instead motivated by the stick, see if you can tap into how you are losing or missing out by leaving the role, and utilize something visual to remind you of that. 

Moreover, some individuals experience shifts in interests because they have ADHD. Of course, one can't diagnose another without having a session with them. In this case making an appointment with a psychiatrist for an assessment may be worth one's time if the other possible answers do not land as helpful insofar as one is open to medication. Alternatively, neurofeedback can also help an individual with ADHD issues, and may be worth looking into.

With that said, there is something lovely about surveying various opportunities. It can really enhance one's appreciation for what is out there. My hunch is however, that many of those opportunities may take more than a week to appreciate as such.

Best wishes on your journey. You deserve to have compassion for you as you walk through it, and reward yourself for aligning with your values as you do.