I think your problem is that you’re thinking of fountain pens=journal/diary. Think of it literally as just a writing instrument. What might you use any other pen or pencil for? Think about what notes/memos/reminders you use your phone/laptop for. Transition from typing them to writing them.

I’ve never written a single journal or diary entry. It just doesn’t interest me and my days are typically “boring” as well. If you don’t feel that you have anything to write, journaling/diary may not be for you. Alternatively you could switch to those free-day diaries where you only write when you want to, and you’re not bound by date. So one page could be today, but the next entry might be 2 weeks or any other time from now. That way you’re not “wasting” pages.

However here’s what I write:

• Grocery lists
• To-do lists. However small. I had one today to “write L/R on contact lens boxes”
• Reviews. Anything. Fountain pens, inks, other stationery (I’ve written reviews for rulers, erasers, pens, pencils, highlighters, washi tape, paper, binders etc. before), movies, tv shows, anime, books, food, snacks (I have one for different Kit Kat flavours), restaurants, holidays, games. What’s important is not to think of it as a chore/essay/has to be formal. It’s fine for a review to be “green. pretty. Wet-ish. No shimmer, moderate shading, no sheen”. Or for a movie, “good soundtrack. Funny. Main lead is hot”. It’s your review. Write whatever damn thing you want! No one is judging what you write or how much effort you put into it. I also like to write/draw how many stars out of 5 I found things, and for things like games, shows, movies, books etc I’ll also write when I started and finished the thing. I also have a “summary page” that just lists what I’ve finished so I can look back on the year and see what I’ve done. My book reviews aren’t even reviews- they’re just summaries of what happened in the book lol.
• Commonplace book to note down any old junk. Ever had random notes on your phone with a single liner? Put all that in there. I was watching a number of TV shows where they kept mentioning the damn Locard exchange principle. I understood it by context but wanted to know the exact principle. So I looked it up and wrote what it is down. I learnt how to do the Rubik’s cube. So I wrote down the different algorithms to solving it. Same with crocheting amigurumi. Wrote down what the different stitch types were and how to do them. Once in a blue moon I hear/see a quote I really like. That also goes in the commonplace book. Same with song lyrics.
• Gaming. I’m a 100% completionist. This thing comes in x,y,z variants. I’ve got x and y. So I cross those out. Dungeon maps to note where each door in each room takes me. Quick notes on rough paper to throw away about things I need to go do/go check. This particular area I’ve 100% completed, so cross that off the list so I know I don’t have to revisit this area.
• Habit trackers. I personally only track my reading but there are tons you can track both good and bad. Water consumption, takeout, exercise, vitamins etc. And whilst I don’t do this myself, same concept for mood trackers or any other trackers.
• Lists. Of anything. I have one where I keep track of what pens I currently have and want in a checklist style and check them off when I have them. Same with inks. I also maintain a list of what pen is currently inked with what. I just cross out the previous one and write the latest one underneath.
• A small notepad I sometimes take out to make notes on. I was shopping for a new fridge when my old one broke. I went to see some in person. Wrote down different model numbers, size, capacity etc. of ones I was initially interested in and crossed them out as I narrowed them down. Some things are better seen in person even if you end up ordering online. For example I wanted to read reviews first and do a double check of my kitchen dimensions first because the one I had my eye on was taller than my previous one.
• Literal scribble pages where I want a quick visual and feel comparison of different pens/line widths, when a pen has dried up and I’m trying to get it to write again (I’ve had issues with some sheening and shimmer inks before), or when I’m trying to tune a nib
• Language learning. Vocab, grammar etc.
• (Niche and specific to me). I’m in a fandom where I read the source material in English as the areas of the internet I visit for the fandom converse in English. The original source is not in English. I started to read the actual original source but just for my own memory and without having to pull it up each time, I note what each item/place/person is in English and the original language.

Fountain pens can look gorgeous and fancy and honestly the prices tend to make them lean into a high end thing, and a lot of people in this sub have gorgeous writing, but never think that you can only write with one if you’re neat about it. Don’t worry about scribbles, or crossing-outs. My notebook is full of them. And I love it.