You can get that feeling back, and usually you can do it without changing jobs. I've helped people make just this kind of change. The trick is, it's a mix of 1) abstracting the fun parts of the enjoyable job such that the individual elements can be spotted and "farmed" at the current job and 2) setting boundaries with and pushing back on any elements that compete against this process. (This is apart from work-external considerations, like having things to look forward to every day after work, etc.)

In learning to do this, people become more motivated than they would be if they just changed jobs. Patience, analysis, and cleverness are required but those skills when strengthened also tend to make people feel like they're in control of their lives again.

Believe me, you're not lamenting about growing up. Look at the details and it's probably closer to a situation that could genuinely use some rebalancing in your favor. Your career should be supporting you and not vice-versa.

Happiness is not something you find, it's something you make when you recognize what is missing in your life and you make that thing part of your life. It sounds like you're halfway there if you've identified what did make you happy.

If you enjoy your job, you're still well ahead of most people there too, even if it doesn't make you happy. Use your spare time to do the things you miss about working in the park, get outside, help people, etc. Maybe you could start a meetup.com group to give hiking tours of local trails/parks. Talk to a park ranger and find out if there's anything you can do with your group while in the park that would help the park.

Happiness is exactly the opposite. It's realizing that nothing external or material can leave you perfectly content for any length of time because that violates what it is to be human. Happiness is a choice you make and a mindset you adopt.