"私の最も厳しい戦いは、最初の妻とでした。"

Simplified Meaning:

The speaker is saying that the hardest challenge they ever faced was dealing with their relationship with their first spouse. Think about how sometimes, personal conflicts with people close to us, like a spouse, can be more difficult than facing problems at work or with strangers. For example, if two people who love each other have very different opinions or habits, it can lead to lots of arguments and stress. These kinds of fights can be very emotional and hard to manage because there is so much personal investment and feelings involved. The speaker may be highlighting that dealing with emotional struggles and resolving conflicts in a marriage is often harder than other challenges in life, like a professional rivalry or a physical fight. To apply this wisdom, one might look for healthy ways to communicate and resolve conflicts with loved ones, remembering that these personal battles can be tougher than they seem and require careful handling.