Replaced left crank arm but still comes loose?
A loose crank arm can damage both the bottom bracket and the crank arms. In your case, since you've run the crank arm til it fell off the bike, it is likely you ruined the bottom bracket. If your crank arm is loose, immediately stop and tighten it.

You will need to likely get a new bottom bracket and possibly yet another new crank arm. Replacing a bottom bracket requires a special tool (bottom bracket tool) and a compatible bottom bracket (which will typically be marked on the bottom bracket itself). You may want to leave this job to a bike shop.