I'm guessing that you have a mid-2012, 13-inch, MBPro?
That would have the built-in DVD drive (and the last model that can have an internal hard drive)
So, assuming that's what you have, the battery may be swelling (causing pressure from underneath the trackpad, and giving you erratic clicking, etc.
It's easy to take the bottom cover off, then remove the battery to check for swelling (it will be obvious to the eye!)
And, you can test the trackpad while the battery is removed, just to see that your trackpad works better without the battery... If that's the problem, all you need is a new battery.