We have slammed against our share of walls. This is using a metaphor, where "walls" mean life's obstacles, difficulties, or barriers to what we are trying to accomplish. The word "share" just implies that everyone has these obstacles in life, perhaps some more than others. If they were to be distributed fairly, or evenly, to everyone, the writer says we have experienced our fair share.
This is a great quote actually. Thank you for sharing it! I hope my explanation helps you.