Very bitter avocados
So first, you should be aware that there is an avocado shortage this year, and as a result the avocados you are getting at the store are probably lower quality than you are used to.

There are two reasons for avocados to taste bitter: underripeness, or because they have been cooked.  If the avocado is fairly firm, then it's probably just underripe and you can wait.  If it's soft, though, then either it was picked very underripe and never lost its bitterness, or at some stage it was allowed to get really hot during shipping (possible since the shortage was triggered by high temperatures in California).  In either of those two cases, it is ruined and you should compost it.

There's nothing you can really do to get rid of that bitterness.  Contrasting flavors, such as tartness from lemon juice, will help cover it up, but it will still have a bitter aftertaste.

References:


https://www.quora.com/Does-avocado-turn-bitter-when-heated
http://blog.homemade-baby-food-recipes.com/why-would-an-avocado-taste-bitter/