Do you want a happy life, or a worthwhile life? I think they're probably correlated, but I don't think you can guarantee one without risking the other. In that light, optimizing for happiness seems like the wrong move.
It's a cliche, but a lot of the unstated reasons we were happy at one time were there all along, until they start to slip away (social connections, health, etc.)--usually without realizing the extent this has happened until years later. As a rule of thumb, I'd suggest that the most important thing to nurture over the years are your friendships and social connections (even if that means just a chat message every few days to a close friend from college who moved to another city), which are probably your best shot at adapting your new life situation to feel more at home with happier moments in the past, but which also look forward to the future (after all, friendships are more than just shared memories--they're comprised of living, breathing people, and as such can lead to new experiences and connections that lead to even better things than just holding onto what once was).
Finally, if you really think about it, maybe the easiest way to be happy is really the satisfaction you get knowing you've brought genuine happiness to someone else. :-)
For example, if you work hard every day at something that reduces the suffering of others, it can be worthwhile, even if in the moment it’s painful.