Best advice that helped me:
Do something you've never done before every single day and keep a record of the outcome. Can be tiny - trying a new flavour of fruit tea or walking down a street you don't know. As the habit forms, make it a bit bigger and start to include some of those things you've kept meaning to do or been curious about. (Like visiting local landmarks or trying a new hobby or exercise.) What i like about it is, you don't feel any pressure to commit. It's not 'You did Zumba once and then never went back - oh you lazy failure' mindset. It's 'Yes! You tried Zumba and it wasn't for you. Good for trying it out. What's next?'
Keeping a record of the new experience is important. It's quite fun to read back over it all. You could keep notes in a notebook. I had a private blog. I would read through it sometimes and realise all the things I had done. I think it helps rewire the neural pathways, so you get out of that depressive loop and lethargy. I really should do it again. I did it for six months and it was life changing.
Other advice I loved: make sure each day you have a tiny thing to look forward to (favourite TV show, long hot bath, home cooked food, feeding the birds etc); each week a small thing to look forward to: coffee/lunch/walk with a friend; craft or exercise class, cinema visit with partner etc; each month a medium thing to look forward to - a weekend away or fun day out, a trip to the theatre, comedy gig, concert or an exhibition; each season a big thing to look forward to - a holiday, a festival, a workshop in something really important to you, a visit to old friends, a fitness challenge like a really long hike; and each year, an action that feels like a major achievement - it could be passing an exam, training for a marathon, losing weight, getting married, seeing DC graduate, moving house, redcorating your home or overhauling the garden, writing a book, putting on an exhibition or performing in a concert etc - something that you can look back on with pride, or something that has been a long-held bucket list - like finally visiting somewhere you have wanted to go to since childhood.
Also - create tiny rituals that make ordinary days pleasant. Mine are: feed the wildlife every morning then make a fresh coffee with frothy milk and watch them come and eat. I often put on very gentle piano music while doing this. It is such a peaceful start to the day. And put on uplifting music that you really love - maybe from when you were in your twenties (or maybe new artists you have listened to as part of your try something new experiment) while cooking fresh food for dinner every night. I get 30 minutes of lovely music every evening.
And the basic, classic advice: take Vitamin D and B-complex supplements every day if you have low mood. Maybe iron too if you get tired and lethargic easily.
Take regular exercise and try to ensure some of it is outside everyday, preferably in nature, even if it's just a park or recreation ground. Put down your phone, get off screens, and read books made of paper - poems, essays and short stories if your concentration is bad. Or go for a short walk after dinner with your partner. just around the block to look at the moon and stars and have a quiet chat.