Ah, OK - an older trackpad... (that would be a trackpad - not a mouse
)
Probably your trackpad settings.
Does it make a difference if you turn OFF all the settings for clicking? That does mean that you can't tap on the pad to select, but the trackpad button will still work the same.
I always turn that off, because my fingers too easily make accidental clicks.
Do you have the "Ignore accidental trackpad input" off, or on?
If your trackpad doesn't behave, even after trying the different settings, you should try a different user account (make a new user just for this purpose) to see if the trackpad is a software issue, and the problem exists on any user account...
Another possibility is to take your main battery out, and check for any swelling in your battery. The reason I ask about that, is that a swelling battery (not unusual on MacBooks) can put pressure on the trackpad from underneath, making it intermittent, and often making the trackpad button hard to click.
That's just something for you to look at - it might not be an issue, but ...