For the glass half-empty kind of person, or if you just want to develop a more positive outlook in life, this app will help you keep a record of three things that went well each day and help you develop a more positive mindset.

Based on following positive psychology paper:
Fleming, Andrew W., "Positive Psychology "Three Good Things in Life" and Measuring Happiness, Positive and Negative Affectivity, Optimism/Hope, and Well-Being" (2006). Counselor Education Master's Theses. 32.
https://digitalcommons.brockport.edu/edc_theses/32

Abstract:
"Positive psychology is the study of human strength, resilience, and optimal human functioning. The goal of positive psychology is to make people happier by understanding and building positive emotion, gratification and meaning. The constructs of happiness, hope, optimism, well-being, resilience and flow are examined in how they relate to positive psychology. The "three good things in life" exercise was implemented with participants and participants completed pre and posttest measurements on happiness, positive and negative affect, hope/optimism, and well-being. The "three good things in life" exercise consisted of having participants journal each night for one-week three things that went well that day and why. The results suggest that the "three good things in life" exercise may increase happiness and optimism/hope. Results also showed that the satisfaction with life scale scores remained the same, a decrease in positive affect scores, and increased negative affect scores. Overall, the changes in the mean scores were small. Implications for the results and further study are elaborated on. The positive psychology field can benefit from further study to examine where its developments can be implemented successfully and where it can be further enriched."