I think I'm happy despite lots of things being very far from ideal in my life. (Won't go into those.) I used to be depressed for years but feel pretty constantly fine these days, except for when there is good reason not to. I taught myself ways to increase happiness. Such as:
I get a massive amount of happiness from ordinary everyday things. Take real pleasure from a good cup of coffee, a cuddle with the cat, the sight of sunlight through autumn leaves, watching parakeets and foxes in our garden, a walk with a friend, exercise classes, hearing a lovely piece of music on the radio, watching a stranger's baby or toddler grin or squeal with delight at something.
I create small, doable projects for each season of the year, so there's flexibility in when they are done. Then I do them. Stuff like redecorating a room in the house, or taking on extra work to pay for a holiday or doing Dry Jan or C25K etc. I write them down at the start of the year or the season and get a lot of pleasure from reviewing them and seeing what I have achieved. Even if they are tiny - like tidying kitchen cupboards.
I aim to have one tiny thing a day to look forward to (meeting a friend or doing a favourite exercise class or even something good on TV) one small thing each week - going on a hike or to the theatre or a get together with old friends or family, a bigger thing each month - going to a gig or away for the weekend , and even bigger thing each season - a holiday or renovation project, climb a UK mountain etc and a big milestone achievement each year (run a marathon, get a book published, move house, get a degree, have a baby etc - anything that makes you feel like your life is changing and progressing.)
Do new things. I haven't done this for ages, but when I feel meh about life, I make myself do something I've never done before every single day for at least a month. Start small - try out a new cafe or walk down a new street or buy a different scent of shower gel. Listen to podcasts by people you don't agree with, tune the radio to a station that plays music you don't usually choose. Keep upping your game until you are doing things you never thought you'd do. Don;t worry about it making you feel instantly happy, just do it anyway and keep a note somewher ein a notebook or on your phone or laptop, of which of the new things you enjoy and would do again.Then make sure you include them in your regular life.
Other good tips are: accept and forgive yourself for your faults. I have ADHD, I spent decades berating myself and feeling ashamed of always losing my keys, my purse, forgetting people's birthdays etc. Now I know I can't help it. I would give anything to be born organised, and I hate that it drives other people around the bend and they think I'm flaky but I also know that it is a processing disorder in my brain and really, I can't help it. So I no longer get angry with myself about stuff I can't change, and that is very liberating.