The next time you buy an avocado, leave it whole, and do not put it in the fridge. Leave it on the counter. It is ripe when it’s slightly squishy to the touch. If it’s very squishy, it’s overripe but still edible. Don’t cut it until you’re ready to eat it. I’m not sure what will happen to the one you have now since you cut it, but it will not ripen in the fridge.