There are always multiple facets we overlook when met with a dilemma, and it is human tendency to initially gravitate to and focus on its negative aspects. In your specific situation, I recommend another view— though you have broken up with a girl who means very much to you, this act has simultaneously allowed you the room/space/time needed to focus on improving your mental health situation. This is priority #1. Forget about everything else for now— relationships, academics, work, etc. Focus on helping yourself first, because if you aren’t able to help yourself to perform (and not dread) basic daily functions, nothing matters: 1) How could anything else that others do actually help you? 2) How would you effectively dedicate time, attention, and affection to your loved one(s)? Generally, these things are incredibly difficult to do for depressed individuals, and hence, the quality of their romantic relationships take a drastic toll (negative consequences include: 1) Permanently damaging the chemistry with the loved one. 2) Potentially affecting the loved one so negatively they begin demonstrating depression symptoms of their own. 3) Becoming collectively trapped into a toxic relationship.).
I know what you mean by saying things you do not mean— often, you will lash out as the “depression talks” due to the mixture of frustration, hopelessness, and chemical imbalances of the illness.
On helping yourself, start with your physical health. This is much more straightforward than overthinking and over-analyzing how to “fix” the mental aspects of depression. Start very small with eating properly again, brushing your teeth, and forcing yourself to sleep at optimal hours. I am assuming you have a nonexistent routine in all of these aspects, as physical health is the first thing to go down the drain when individuals get depressed. Trust me on this— force yourself to exercise here and there to release your body’s natural antidepressants. Consider getting treatment afterward, though this is all trial and error.
Regarding the situation with this girl, once you are healed physically/mentally, if you still feel the same way, give it a thought to pursue her again. However, do not let this single dream deter you from seeing any meaning in life anymore, because there are too many aspects of the future which are simply un-anticipateable. Assuming you are younger than 38, you have more than half of the rest of your life ahead of you, meaning there is a statistical possibility that you will experience a better version of everything you know (you may have read this 50% rule through one of my Quora posts).
Message me here or through Quora anytime.