The first sentence is about feeling. You should know how the person you like feels after being with someone else, or the change in your heart that you have to give up. It will mature all of a sudden.

The second sentence is easy to understand. When you have reached your pursuit, you will no longer pursue it. It is the end of your pursuit. When you have found it, you will no longer look for it.

The third sentence is just a very realistic statement, confirming and affirming that his feelings for his lover are love, or that his feelings for his lover have reached the extreme, but due to various realities, he cannot be with his lover.

I don’t know the background of these sentences, but this is how I understand them