How do I keep my emotions in check? I have major mood swings that are detrimental to my mental health.

Many times the moods that we think of as spontaneous or irrational have underlying reasons so a good thing to try would be trying to identify the patterns. Try to find patterns in your mood swings, think about what emotions they are and in what circumstances they arise. Also, try to see what helps dissipate them. All of this can help you understand more your feelings and develop a plan of action for when those emotions come giving you a better sense of control. Also, remember you are human and that emotions are normal, when you feel like you are having (or about to have) one of those major mood swings acknowledge that you have the feelings and that that is ok, give yourself the time to rest/breathe the feelings away by doing something that will destress you, the same way you'd rest if you were physically feeling unwell. Remember it is ok to ask for help because when things are overwhelming you shouldn't be expected to go through it alone.