I'm having trouble sticking to my spiritual practices. Is there a story in the Satcharita about a devotee with a similar problem?

Yes, the story of Balasaheb Deo in Chapter 41 of the Shri Sai Satcharita directly addresses this kind of struggle. Deo had a deep desire to read the Dnyaneshwari every day but found that some obstacle would always prevent him from doing so, even though he could read other spiritual books without issue. He became so distressed by his inability to maintain this practice, even failing to read just five verses daily, that he made a vow. He decided he would not attempt to read the Dnyaneshwari again until he received a direct command from Sai Baba to do so. This story shows how Sai is loving toward devotees and helps them overcome difficulties in their spiritual path.