Your character's internal struggle is the third rail of your story - the electrified rail which propels your story forward.
Without that third rail your story, like a train will simply sit there at the station going no where, it can look pretty, be very well constructed, beautifully painted, but it's not moving anywhere and stories like trains must move forward.
This is one of the biggest reasons manuscripts are rejected, because the story is polished but it's just a bunch of external events which happen around the characters, they're not dealing with, and changing the internal beliefs of the protagansist.
The story should be about the protagansit being broken, having an internal problem, a misbelief about themselves and/or the world and the story is about their journey to fix that. The character has to learn to overcome, to deal with that internal problem before they can defeat the external challenges the plot poses. That means the internal problem will often predate the external problems posed by the plot, often by decades.
You need to know what your protagonist wants, what internal belief is in their way and more importantly why, in order to construct a plot which will help the protagonist overcome that problem.
I'm writing this blog post because I recently had this issue with a piece I was writing. I knew the issue was there, I'd read Lisa Cron's book - Story Genius, so I knew about the third rail. My story was about an abduction and a law enforcement officer who was responsible. Originally I wrote the story all about how her personal integrity was at stake, how the organisation's reputation was in jepardy, how a child's life was at risk. I figured that was enough to drive the story. But these are mainly external plot issues. Then I had the Eureka moment when I realised that the protagenist's father had always said she wasn't up to the job, he didn't respect her, didn't think she could do it, this has lead to her having doubts herself - imposter syndrome. Suddenly I could see the electrification of that third rail. I just had to plant a couple of seeds of internal self-doubt and the story took off. Suddenly the story was about her relationship with her downbeat father who never supported or encouraged her, infact doing the opposite he ground her down every chance he got.
"Many of our deepest motives come, not from an adult logic of how things work in the world, but out of something that is frozen from childhood."
- Kazuo Ishiguro
I came across the above quote this week and I think it actually sums up where that third rail electrification often comes from. Childhood is often the source of these misconceptions which need overcoming and that provide that power to propel your story forward.