alternator belt keeps breaking

You can buy a better high quality belt than the cheaper standard belts. The alternator is attached to the engine if its moving around because a motor mount is bad would not cause this unless its rubbing against items under the hood that remain stationary. I would check alignment and pulley condition and being over stretched upon first installation of the belt.

Check the condition of the alternator being hard to turn it should be easy to spin the pulley.

Check that all pulleys are easy to spin and are not loose or wobble

Check the crankshaft pulley. It may be coming apart tossing the belt off due to misalignment.