Completely separate internal and external conflicts
Question: I have a story that has the main character in a battle against some evil people. They are his external conflict. At the same time, I have him in fear of falling for a girl because of inner conflict that does not relate to his external conflict. Is this ok, or is it to be avoided?Answer:
Often writers will create similar types of problems in different throughlines as a way of binding the story together. It is a useful technique, creating a kind of irony. Often they do this unconsciously just because of the ideas buzzing around in their head while they write the story.
It's also true that the resolution of the main character's inner conflict should (not must, but should) determine whether he is able to achieve the story goal. That's one of the strongest ways in which the main character and the overall story connect.
For instance, if the girlfriend is the impact character, and if the main character decides to change, that would mean he follows her example and adopts a new way of being or doing things that helps him defeat the evil people (assuming you want him to succeed).
Whether their relationship works out in the end is another matter, of course. He could change, defeat the evil people, and still lose the girl.
The fact that he doesn't know ahead of time what choice will help him achieve victory, whether to follow her example or not, creates the uncertainty that makes for a suspenseful crisis.
Of course, not every story follows this structure. Sometimes, if your passion strongly advises you to go another way, that may be the best choice. This is just how good stories typically work.