The first thing to check is the basics.
If you are eating shit food and are poorly nourished, barely spend time outdoors, have a literally toxic environment from industrial, are surrounded by draining and miserable people, then it will be impossible to feel good.
It's not going to hurt you to take a multivitamin and occasionally go on megadose regimes of vitamin c, b vitamins, or d. The FDA recommendations should be regarded as bare minimums - in some cases you want to be aiming daily for 2 or 3x times the given amount. Routine physical activity, fresh air, sunshine, and nature help a lot and should all be nonnegotiable features of your life. Depressed landscapers are exceptionaly rare. Plants have been shown to improve moods of people, so having them around in your space doesn't hurt either.
Within the biochemical frame, also look at the most basic thing. a lot of mood disorders like depression come down to dopamine and serotonin. Engaging in diet and activities which mind this, especially over the long run, can be a key to success and basic levels of happiness. Again diet and supplements, exercise, and sleep, all come into play here.
A cognitive lens would suggest looking into the way you look at things.
There is more and less useful/advantageous ways of frame things. Be mindful of that. Likewise, a lot of anxiety stems from 'should': expectations which might not align with probable reality as it exists. Dealing with past trauma would also be a feature, if there is a specific issue. I'd avoid 'therapy' in which you endlessly examine the bad. Rather, it's important to develop specific cognitive frames which are both useful long term and serve as a sort of emotional regulator to life's inevitable issues and features. A basic cognitive framing question would be, 'do you see life as a series of things that happen to you or for you,' for example.
A certain existential school might say you need to create and surpass reasonable challenges to transform yourself into something anew, as well. Learn something new, face a big, somewhat absurd but doable long term physical challenge. Again, you're probably not going to regret learning a language, doing a marathon, joining a boxing gym, or trekking somewhere. More likely, there's something uniquely personal that you've wanted to do for a while but have held off on, and in pursuing that goal, you'll find a solution to the existential problem (opportunity).