One way thats helps me is keeping a diary and writing EVERYTHING in it e.g past trauma's, every day to day life and everything that slightly bothers you. If you can speak to someone about everything thats even better if you cant then pouring out all your negative emotions onto paper helps.
Second thing is getting a plant or a bird feeder outside your bedroom window. Taking care of something e.g a plant can really improve your mental health. I suggest getting a oxalis triangularis because it only needs a lot of sunlight on a window sill, watering once the soil is dry but do research.
Getting a bird feeder can really help your mental state because watching cute little birds outside your window eating mealworms really helps lift your spirits.
Finally, changing your attitude to things can really help. For example, if you left a wrapper on the desk and think "eh, ill do that later" it is an okay attitude but instead of thinking about doing it in the future, you should just get up and do it so its out of the way. Instead of thinking about doing things in the future, think about doing things in the present. ALSO going out more even for little things e.g a little walk around the garden and just spending a little bit of time outdoors even if its for five minutes can really help your mental health, especially if its with friends.
I hope this helps!