There are many different examples of internal conflict within Shelley's Frankenstein.
By definition, internal conflict is a disagreement or battle which one fights with ones self based upon morality and personal belief.
There are three types of external conflict in which man/woman can find themselves in 1. man v. man (society); 2. man v. nature; 3. man v. supernatural. With each of these external conflicts, man finds himself battling with other people or society, against nature (storms, natural disasters, animals), or inhuman beings (ghosts, demons, mythological beasts).
Internal conflict is not broken down in the same way as external conflict is. Therefore, Victor's internal conflict can be made against nature and society, but the conflicts are ones in which he struggles given his morals and personal beliefs.
Victor struggles internally with nature given that it is not the "job" of man to birth another. God has given women the power to give life- not men. Based upon this, Victor's obsession with reanimation and the death of his mother fuel his desire to create life. This is unnatural in the world- for a man to create life. All of science goes against this idea. The internal struggle which Victor faces with nature is the fact that he should not, given he is a man, be allowed to create life.
My internal being was in a state of insurrection and turmoil; I felt that order would thence arise, but I had no power to produce it.
After days and nights of incredible labour and fatigue, I succeeded in discovering the cause of generation and life; nay, more, I became myself capable of bestowing animation upon lifeless matter.
Here, a reader can see Victor's obsession with creating life- something which nature had not intent of letting a man do. Victor's internal conflict with the fact that nature allowed only women to create life became a conflict in which he would resolve.
As for internal conflict which Victor felt with society, Victor feels that he must protect society from the monster that he has created:
Shall I create another like yourself,
whose joint wickedness might desolate the world!
Knowing what the monster asks, to create a mate for him, Victor knows that he cannot doom the world (society) by doing so. Unfortunately, the monster gives him no choice. Victor must create the monster a mate or face losing Elizabeth. Victor's internal conflict revolves around the fact that society is a greater whole than Elizabeth, but his love for Elizabeth rules him. The conflict erupts from his desire to protect society initially, but his love for Elizabeth surpasses his love for society.