I would not use "to get along with" in this sense – the verb only makes sense when applied to another person (for example, "I got along with my friend"). If you want a phrasal verb that means "to find something easy", I would use "to go well". Then the first sentence would be "I felt depressed when my schoolwork didn't go well" and the second would be "I felt depressed when my schoolwork wasn't going well." Both sentences sound fine to me, but I think there's a slight difference in meaning. The first suggests that I potentially still felt depressed even after I wasn't working on my schoolwork anymore – for the rest of the evening, maybe. The second sounds more like I felt depressed through the process of working on my schoolwork but not after that. The difference is very subtle and other native English speakers may not agree with this! I hope this helps; the difference between the past progressive and the preterite is difficult for me to put into words.