don't know of anything online specifically for riding, but i have kept riding journals in notebooks, word documents, and googledocs, and all worked well for me. you just have to be dilligent about updating, and be able to create a format that works for you. i would write down the date, what i planned to work on, what i actually ended up working on, how that went, and goals/reflections for my next ride.
good luck with it! keeping a riding/training journal definitely helps me stay focused.
There is a training tab and I just create an entry for each time I work with my horse. I also use their care tabs to track everything else and then I get emails that it's time to worm or trim feet or get spring shots.