I've never really thought of long term happiness as attainable. Fleeting joys like food, sex, achievement bring a certain amount of dopamine/serotonin/norepinephrine. It might be helpful to get sunlight in the mornings and evenings, as well as eating foods that have precursors to those chemicals to make sure you receive the proper building blocks to feel good emotions.
Apart from that, I found that letting go of the outcome and desires has been really helpful. I'm trying to stop being so immersed in life, and I just observe it. I observe my emotions, whether they are good or bad, don't place judgement and try to let go. It takes practice and skill. Lately, though, I had some moments of calm and peace and I felt really happy. It's like when your mind clears and you just feel ~good~ for no reason at all. I think it's important to not chase this feeling.