Habitica is the best productivity app I've used thus far. It lasted me for months. The quests are the most motivating feature, and I was able to take down a couple bosses solo. However, motivation dried up weeks into my first fetch quest because despite being extremely productive, I had no chance of finding even one shard. Unless you're comfortable teaming up with a bunch of strangers to take on quests, there becomes a point when being solo makes the quest motivation feature impossible.