The conflict of man versus self is when a character is fighting issues within himself. The most prominent example of this in Of Mice and Men is George. George is an upstanding man who made a promise to take care of Lenny. He truly loves Lenny, but keeping Lenny out of trouble is taking a toll on George. George wants to be free to live a normal life and be successful, but Lenny tends to hold him back because he is always messing things up. Throughout the book, George struggles with his conflicting feelings about Lenny. He cares about him and he wants to take care of him, but at the same time, without Lenny, George could be free to live the life he wants. When Lenny accidentally kills Curly's wife, George comes to the realization that there is no saving Lenny this time. Running away, like they have in the past, won't work. Because he loves Lenny, George kills him so that no one else can. He takes Lenny's life in a moment of peace for Lenny.
You can also see an example of man versus self within Lenny. Lenny wants to be good and do the right thing. He really tries, but ultimately, he knows he can't truly control himself. This is why he consistently looks to George for reassurance. He recognizes that what he wants and how he acts don't mesh.
Conflict is all over the place in the novella, Of Mice and Men. One type of conflict is the conflict within a person's own heart. To put it another way, the world of Of Mice and Men is so filled with conflict that it is also internal in a person's heart.
The most pronounced example of this is George. On the one hand, it is clear that George loves Lennie as a friend, perhaps even as a brother. Even though people dismiss Lennie as a fool, George stands by him. On the other hand, there are times when George cannot stand Lennie and berates him. Within George's heart there is tension. Should he be faithful to Lennie or just let him go his own way?
At the end of the book, Lennie accidentally kills Curley's wife. When this happens, George, conflicted, decides to take Lennie's life. He struggled within, but he made his choice.
We can look at other characters to see conflict within. In particular, you can see this dynamic in Candy, the old swamper and Crooks, the lone black man on the ranch. We can also see this conflict in Curley's wife.