There is no need for a proprietary data format in 2018 anymore. Using plain text as the basis, organise your files in folders if you like, encrypt and sync using some cheap (or free) cloud storage provider, and on top of that choose your favourite editor (I use iaWriter because of nice UI + the ability to simply drag images into a file).
You may get hooked on keeping a journal (as I have - one of the best decisions I made) and you will not only amass a lot of text (I have written the equivalent of 4000 book pages) but also want to do this for a long time (speaking decades), far longer than the life-span of most if not all journaling apps.
If you use a proprietary format, at least make sure that the app has a decent export function that creates a result that doesn't require manual changes by you.
I do the same thing, except that I recently made a simple editor UI that shows me the entries of the same day of previous years when I'm writing today's entry.
I've only been journaling this way for a couple of years, but for me this is a good way to review how were my life and thoughts in the past. Reading previous entries gives me more chances to reflect. Many times I feel I've grown, many other times I detect I still suffer from the same issues and this prompts me to take action to correct them.
You don't need a UI that does it, but I suggest reading back some of your previous entries from time to time.
It has a ton of functionality that I don't use, but it's reliable, has effective syncing across devices, and a clean export process. There may or may not be better, but I'm very happy with it.