One exercise I recommend is writing yourself a handwritten letter when you are in a good mood. In this letter, talk about the smallest thing/s you are grateful for (flowers, trees, food, etc..) and also bigger things you are grateful for (a pet, a child, a grandparent). See if you can stretch yourself to come up with these things.
Next, write about what you like about who you are and what you are and things you are proud of in your life. You may feel like writing about how hard you are on yourself and how you want to try to be kinder to yourself. Try to write the letter in a way that when you read it later when you feel low, it will make you feel better. You may end up reading it often.
If you are in a dark place where you can’t write this letter, pretend that someone else is writing it about you in a time you felt better and extracting a positive light you cannot see because you have blinders on at this time.
You can even mail yourself the letter some random day, if you want, or put it away for a difficult day.