It happened the most while attempting to study research papers.
A lot of times I would be unable to finish reading what I had planned, because I had stumbled upon an interestingly-looking concept and then proceeded opening up another paper on that subject and so on.
And then, of course, having returned back to the original paper, I would have to re-start reading from beginning to freshen up my memory.
Terribly exhausting process, so I can perfectly understand your frustration.
The only solution I found was to un-clutter my computer 'work-space' as much as possible: close any non-essential apps, unplug internet cable and each time I have the urge to stop reading and go research a newly discovered subject, I remind myself that I am only allowed to do it after finishing what I'm currently reading.
Another helpful thing for me is to remind myself what I'm trying to accomplish with whatever I'm doing at that particular time and what my long-term goals are.
For instance, if I'm working on a project for a client, the goal is to get the work done as soon as possible and obviously get paid. I am not working on said project to primarily enrich my knowledge, but to make money.
I can use the time after the project is delivered to draw conclusions from the experience or do further research.
This may sound trivial, but it really does help to constantly remind yourself of what your goals are, it keeps you in check.
For extra effect, every time you have the urge to let your mind wander too much, try imagining the possible consequences of not completing your task (on time).
This can be particularly effective if you're doing client work. Imagine how embarrassing/unprofessional would have to explain to your client/employer that you won't be able to deliver on time because of something that you could've prevented.
Side-note: you have not provided enough info, but you may have obsessive-compulsive disorder, which is nothing to be ashamed of, but you can only 'solve' this with medication, so you would need to see a doctor.
It happened the most while attempting to study research papers.
A lot of times I would be unable to finish reading what I had planned, because I had stumbled upon an interestingly-looking concept and then proceeded opening up another paper on that subject and so on.
And then, of course, having returned back to the original paper, I would have to re-start reading from beginning to freshen up my memory.
Terribly exhausting process, so I can perfectly understand your frustration.
The only solution I found was to un-clutter my computer 'work-space' as much as possible: close any non-essential apps, unplug internet cable and each time I have the urge to stop reading and go research a newly discovered subject, I remind myself that I am only allowed to do it after finishing what I'm currently reading.
Another helpful thing for me is to remind myself what I'm trying to accomplish with whatever I'm doing at that particular time and what my long-term goals are.
For instance, if I'm working on a project for a client, the goal is to get the work done as soon as possible and obviously get paid. I am not working on said project to primarily enrich my knowledge, but to make money.
I can use the time after the project is delivered to draw conclusions from the experience or do further research.
This may sound trivial, but it really does help to constantly remind yourself of what your goals are, it keeps you in check.
For extra effect, every time you have the urge to let your mind wander too much, try imagining the possible consequences of not completing your task (on time).
This can be particularly effective if you're doing client work. Imagine how embarrassing/unprofessional would have to explain to your client/employer that you won't be able to deliver on time because of something that you could've prevented.
Side-note: you have not provided enough info, but you may have obsessive-compulsive disorder, which is nothing to be ashamed of, but you can only 'solve' this with medication, so you would need to see a doctor.