I would suggest that you plan it out on paper. So you said you have the beginning and the end, now you need to think in terms of plot. I don’t know if you already have one, but the next step would be to find a climax. Then if you feel as though you need to fill it in more, add relationships, (not necessarily romantic ones, they can just be friendships, etc., just a lot of characters) and then wide stories and side conflicts can also fill up that space. There isn’t really a quick fix for writers block. You really just need to push yourself- take the time to map things out, look at other stories for inspiration (sometimes it can help if they’re similar to yours- obviously don’t copy, but they can inspire you) if you would like you can also run through what you have so far with me and I can help you think up some ideas. Sometimes going through it with another person can also help 
Yeah, you’re right - I tend to just do all the planning in my head, so actually writing ideas down is probably a good idea. Thanks!
You’re welcome! 
Ahh girl I need your help the writers block is reaaaaal do you have insta if so let’s talk on there here’s mine @Ice.episode_
K! I’ll go follow you right now 
my insta is @anna._.epp
Can you help me think of side stories to my plot? 
Sure! Side stories are basically branches off of the main plot. It seems like you already have a lot of character relationships, I would recommend focusing more on the conflict. You could add in kinds of antagonists, like people who are looking for the journal, or they could be other kinds of antagonists (I.e. psychological) Then the quest is in itself a big part of the plot, so if I’m being honest, I don’t think you really need side stories. You might just want to jump right into the conflict at hand, and then have the characters develop throughout the quest. Your exposition really shouldn’t be longer than a chapter.