Training diaries are good for noticing small improvements, which is where the success and enjoyment in Parkour is.
The other thing that works is to set yourself specific challenges each time you go out to train. For instance, you might see if you can do a jump 100 times in a row without falling or losing your balance. Then, the next time you feel like training, see if you can do maybe 110. When you successfully complete a challenge, make the next challenge a little bit harder. If you have specific targets it's easy to see you're getting better as those targets keep getting harder.
The last thing is to keep reminding yourself of what you are trying to achieve in the long run. Our feelings can be influenced by our thoughts. The things we desire are the things we think about the most. The more often you remind yourself, the more likely the motivation is to stick in your head.
Trying to be a helping hand from NorthernParkour and the British Parkour Coaching Association