Welcome to the forum and thanks for sharing your story so honestly.
I can feel you are upset after reading what your partner wrote.
I can understand this has upset you .
I journal to me is private and I would be upset if anyone read any of my inner most thoughts. A journal is a chance to go through ones ideas and thoughts and try to make sense of them. They are not real and the reason I write in a journal is so I don't say my thoughts out loud and upset people.
He told you the following which agrees with what I said about what a journal is for.
"He swore they’re just thoughts and he wrote them down as he doesn’t understand them and is trying to get rid of them - but he would never ever cheat or entertain someone else which he swears by."
think the fact he wrote these feelings down and has not acted on them but is trying to get rid of them shows how hard he is trying and how much he loves you.
f you can try and think about the fact he wants to get rid of these thoughts, how much he wants to make the relationship work, rather than what you read.
Maybe you could write down how you are feeling in a journal and work out what you can do to see things in a different way.
Keep the conversation going here if you want.
You are both trying and being so honest with each other.