Motivation is almost always external due to its tendency to be coming from a source other than ourselves. Many people associate inspiration with motivational messages. There are many circumstances where something is motivational but not inspiring but these cases are subjective like most.
Self-motivation is distinguished by its source, which is usually within yourself. For some people, the difference may seem arbitrary. The source does make a difference if you consider that all forms of motivation are energy and momentum.
Most of us do not keep track of the things that motivate to the point that we build onto them and keep giving them energy. Some people do keep track of things that inspire them and they almost always reuse the same visuals, messages or anecdotes to inspire themselves until they stop working. Why they stop working is the same reason you don’t eat the same food for every meal, unless you do. Humans require variety and diversity.
To provide a constant, consistent and diverse source of motivation, focus on appreciation, passion and making the world a better place. These things last forever, come from within and change as you do.