Write your sales pitch to Yourself.
For a diary or anything else that requires a long or daily commitment (like writing a novel or exercise or jokes), I would start by writing WHY you want to do it.
What is your motivation?
What do you hope to gain from it?
Try to come up with the reason or two that you find most compelling for doing this thing, spending your time on it.
I am awake an average of eighteen hours a day and sleep an average of six. If I spend 30 minutes writing almost every day, that will be around 180 hours a year, likely an entire month of full time work (at 40 hours per week).
So I ask myself, if I could, would I devote a month of full time work to gain the benefit I anticipate from writing 30 minutes every day? What can I get for that, that will motivate me?
Write that down, a sales pitch to yourself that you can read in about thirty seconds, (about 100 words), to remind yourself WHY you want to do this every day. Then commit to that reading, every day.
Put it on the mirror so you see it when you brush your teeth. Put it on the wall across from the toilet. Put it on the coffee pot. Hang it above your computer screen. Find a way to make it unavoidable, and commit to reading it instead of ignoring it.
If it stops selling you, decide why, and revise it to motivate yourself, or at least decide why this is a waste of your time, why you are not getting what you had hoped to get from the exercise. Nothing says you have to continue wasting your time forever, and people change what they want out of life as their life progresses, and as they learn new things about the world, and other people, and themselves. That is especially true of the young, but can also be very true throughout life, all the way to the old and retired.
It is our human nature to have big goals that will take much time and work to accomplish, but to walk through our lives responding to the urges, demands and fun whims of the hour, thus never accomplishing those big goals. The big goals generally demand sacrificing many hours you could have been enjoying yourself instead.
Write your best sales pitch to convince yourself that sacrifice is going to be worth it in the long run, when your short-term urges are to watch some rerun episode of your favorite TV show for the third time, or surf the net to find something funny.
Then put that sales pitch where you can't miss it, and commit to reading it every day, even if for some reason you haven't followed through in days. Don't just ignore it, if you aren't executing, make a clear decision why, write that decision down to memorialize it and cement it, and abandon the project because you have truly changed what you want out of life.