I used to have problems with low energy, and can give some comments from what I've learned from excesive googling and youtube watching (some books aswell).
Excesive fatigue means you have overworked your body with stress. Might be because you have trouble dealing with stress, or it might be because you have experiences to much stress. Either way the way back is the same, decrease stress (temporarily) and increase your ability to recover from stress.
First of all you shouldn't be training rigorously. If walking stairs tires you, your body needs rest, and any training should be easy. Short term a workout might give you more energy, but long term it will drain you. I recomend no more than 40 min per day at low intensity.
Very likely the most important part is sleep. It's very likely that your sleep is not good. If you sleep enough then it is likely quality is low. You should aim to sleep so much that you can wake up without alarm clock. To improve your sleep i recomend going up and going to bed at regular times. I'd aim for at most +/- 30 min everyday for wake up time, and +/- 60 min for bedtime. If you party a lot, that might be something you have to give up to some extent for a while.
About partying. You should reduce alcohol. Idealy never drinking more than 2 drinks, if you feel worse the day after, that's how you know you drank too much.
It's likely you have deficiencies, stress tends to deplete the body of nutrients, leaving you worse at handling stress. Try to eat extra healthy for now, lots of vegetables and greens, and avoid processed foods. Also if you have the oppurtunity, aim for 30 min of daily sunlight without excesive sun factor. Eating dietary suplements can be good if you know for sure you have a specific deficiency. But there is a lot of research the past two decades about the importance of gut health wich eating vegetables and so on helps with because of fibre, ensymes, antioxidants and so on.
And then you can find ways to decrease stress psychologicaly, but that's to complicated for me to give advice about.
I'll say it again because it's so important. Fix your sleep. Every night should be a good night's sleep. Good sleep can dramaticly increase quality of life and is always worth it.