Stiff trackpad on Macbook air
Your trackpad is most likely bad.  It's not unheard of, after all, it is a mechanical device to some extent.

It could be either it's just worn out or there may have been some sort of liquid spill like juice that is causing it to become "sticky" (I personally repaired this condition). 

There is the possibility that the battery may be swollen (you may have to examine it when the back cover is off; even remove the battery).  If that's the case, you will also want to replace the battery.  I recommend getting one with at least an 18 month warranty.   

You can get a replacement trackpad fairly inexpensively, but it will take some effort - you will have to remove the battery.  Ifixit.com has an excellent step-by-step guide to walk you through.

As you can see in the pictures, once you have the back cover off, you can gain access to the battery.  After you remove the battery, you then have access to the trackpad.




This is not a difficult repair, but detail oriented.  If you take your time, you should be able to accomplish this.  If not, any reputable repair shop can tackle this for you.