Left crank keeps coming loose
I’m assuming you have a square taper bottom bracket + crank arms?

See this answer: Square taper crankset develops play after changing bottom bracket

If you’ve been repeatedly riding with a loose crank arm it could be that it’s now damaged.

You could try tightening with lots of torque, grease on the square taper and threadlocker (e.g. blue Loctite, but in a pinch any kind of glue works) on the threads. A retaining compound (e.g. LOCTITE® 648) on the square taper is even better than grease.

If that doesn’t help, the easiest, surest solution is to replace the bottom bracket and crankset. A completely new Octalink crankset (which doesn’t have this problem) can be had for less than 30€ (+ bottom bracket for &lt;10€).