When my sister was younger, she loved baking and would often rope me into helping, although that usually meant she would bake and I would clean. One day she was baking by herself and was using blue food coloring. She accidentally spilled a bit of it on the counter and didn't bother cleaning it up, it was just a little spill, no reason to worry about it. But before long, the blue food coloring was everywhere. All over the counter, in the sink, across the floor, in the silverware drawer, in the living-room, and somehow on the dog.
One little spill quickly spread blue across the house.
This is the exact same thing that happens in our own hearts when we let unforgiveness fester. Jesus said to leave your sacrifice on the altar and go make peace with your brother not because seeking God and sacrifice isn't important, but because there is a sense of urgency when it comes to forgiveness. As painful and hard as it is, choosing to let go of our anger against others, God, and even ourselves is an essential part of our journey to becoming more Christlike. And when we choose to cling to our bitterness and anger, it's like that blue food coloring. It quickly spreads to other areas of your life, things that seem completely unrelated.
Suddenly, you're struggling to hear God because you're clinging to lies. A person vaguely looks or acts like the person who wronged you and that relationship is destroyed as well.
Instead of choosing the simple (not easy, but simple) act of forgiving, you allow the blue food coloring of bitterness to cover your entire life.
I've experienced this in my own life and am hard at work wiping away every place unforgiveness has touched. I encourage you to look at your own heart, ask God who you need to forgive today.