It may helpful to try to train yourself to think that her retirement is normal and was always to be expected. Just as it is normal for you to have to change your lifestyle for grad school, it is normal for our horses to have good years and then need to retire and change their lifestyle. This is normal life. It has its sad expects that we grieve for, of course! But maybe you could think of both your current chapter and your horse’s current chapter as “the current adventure” (however wryly said). Find some silver lining, however small, and lean on that. Work that silver lining!
Life changes all the time. We surf the wave, not control it.
Many hugs!!