Macbook Pro: one click is registering as holding down the mouse?

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I have the 2007 or 2008 macbook pro. I don't use an external mouse. In the past few months, it has been having this problem..When I click once, it registers as if I'm holding down the mouse, which it SUPER annoying. For example, if I'm reading an article and randomly click once somewhere on the page, then move my mouse, it starts highlighting the words. I don't understand why that happens, because I only clicked once! Not holding down..
Another example: Say I have multiple tabs open. I click on one of the tabs to switch to. I click once, of course, but it thinks I'm holding down the mouse. So it starts trying to move the tab.
Also, if I click once in an open space on my desktop, it normally shouldn't do anything, because it's just an empty spot on my desktop. But with the recent problem, it thinks I'm holding down the mouse, so one click causes it to start highlighting things on my desktop.
This issue is really annoying me. It's almost impossible to write and edit essays I'm writing without going crazy. Because when I'm just trying to click to a different spot in my essay, it starts trying to highlight the writing! Ugh!
Anybody have a solution to this problem? Please!!! Other than this, everything else is working perfectly fine.
Thank you sooo much!