Hopefully this sounds like obvioslus advice but the N3 exam and speaking are very different things. You don't need to be able to apply でしょうがない to have a meaningful conversation. I passed N3 after my first year on jet with no Japanese study before. Conversation still alludes me. But for N3 being able to read cleaning notices, menus and simple emails is enough. So don't get in your head. My trick was to use what j had learned in phone calls or in service situations, even if it sounded out of place. Just so I could use the grammar pattern. Then usually I apologised and moved on. Be sure to practise those に/を/と/が things (honestly I have no clue about Japanese in a formal manner) and how they relate to verbs or objects. But grammar is easier than you may find in the exam so long as you're not already convinced you're bad at it