You've essentially developed a habit. Like others have said, this might be caused by various factors, so look at what's lacking in your life and try to fix that, but afterwards you may need to create a plan in writing to correct the behaviors of the habit that has formed, because otherwise, even if you fix the root cause, the habit might still remain.
As for how? That's a good question that I don't yet have the answer to. I'm working on the first part myself. After I fix my root cause, I think I will fix the habit via a combination of lists and daily resets (i.e. no old tabs on the computer the next day).