Willow73 (and others above) are quite right, in that a focus on something like art, books, crafts or exercise really calms the mind and lifts the spirits.
Having reduced from two to just one dog walk a day, I do it later, before lunch, and stay out longer. Being in nature, even a garden, has a very powerful relaxing effect - as does exercise.
I need to have a sense of achievement and progress to be happy. Therefore I tick something off my 'to do' list every day - even if it's just a ten minute task. I'll phone, text or email somebody every afternoon.
Living alone (as I have, for five years) be sure to spoil yourself any way you can. Special food, treats, long soaks in the bath, watching films and comedies, reading etc. - indulge in your hobbies and/or try new ones.
Having things to look after (pets, plants, garden birds etc.) gives us a sense of purpose and progression too.
I switch routines about, maybe have dinner at lunchtime, shower before bed, get up early and then have an afternoon nap, anything different.
Music really motivates and cheers me, too. It's especially useful to help with a task or drift off to sleep. I'm kind to myself, just as a loved one would be.