That is a problem… I am hoping they will code a search function into Journals. There is a feature request you can vote for if you’d like to tell iNaturalist you are interested in it. Perhaps if we get a lot of votes it can be implemented.
https://forum.inaturalist.org/t/add-a-search-bar-for-journals/24199
There is also a request for adding tags to journals which sounds very similar to me - I don’t know if you want to spare a second vote for this.
https://forum.inaturalist.org/t/add-tags-to-journal-posts/13875
But, until then, there really isn’t a good way. I suppose you could make your first journal entry a kind of an index by month and then keep editing it whenever you make a new journal entry… that would take some foresight, though.