Tiredness in general pertains to a lack of fire, but in regards to studying, this can relate more to Jupiter. Mercury is also related and rules the mind, whereas Jupiter relates more to the journey of discovering something.
Kenaz relates to a drive to study. Sowilo relates to the Sun and can create motivation for anything. One possible opportunity here could be using the Sun in Sagittarius as a working or square, since this blends characteristics of Jupiter and the Sun, perhaps aiding you to see more value in your studies, as well as giving the energy to follow through.
If you have any health issues take note of what these may be, as certain digestive problems for example can inhibit and weaken spleen qi, leading to just feeling very depleted when doing strenuous mental work like mathematics.
This may not be the case for you though, I am just sharing this so you are aware.
Regardless, JoS Guardian Blitzkrieg's post and wisdom on the subject would likely be the number one solution to solve what you are experiencing.