If you’re trying to figure out your main character’s voice or who they are, a journal entry from them to you can help a ton! If you are writing a letter or journal entry then it is easier to capture how they would talk, think and how they would describe things. It's almost like you are the interviewer and are letting them tell you about who they are and why they need their story to be told. If you haven't tried this exercise yet I suggest you do. It can be very helpful.
One thing that I do believe can help you if you are struggling to sit down and write is to do a five minute sprint. You don't have to work on your story if you don't want to during that warm up, but you can. The idea is to just warm up so you can really write anything. Then set your timer for a 10 minute sprint this time working on your book. Slowly increase that ten minutes to the 25 minute mark and before you know it, sitting down to write for a half hour (5 min warm up plus the 25 min writing time) will feel easy, and natural. I challenge you to give it a try today!