I retired on my investments in my early-mid 40s, some time ago. It was one of the best decisions I ever made and I have zero regrets. I've kept busy, occasionally kept touch with work collogues but not really. Work friendships aren't really friendships right?
It obviously sounds like you've made your job, you. Remember, we are not our job or career. You're missing the challenges, the stresses, the mountain to overcome.
This has nothing to do with external forces and everything to do with you. It's not my intent to insult but it sounds like, due to your personality, you're unable to create goals and challenges on your own. This is what it comes down to.
Since your personality depends on external goals, as other have said, you need to join or apply to something. School or volunteering. Buy and old house, learn how to renovate. Electrical, plumbing, structural work, concrete. Flip the house. If you break even, wonderful. If you lose $ on it, who cares since that's the price of entertainment.
Alternatively, if you're able to, set internal goals and CREATE A SENSE OF URGENCY that you need to push yourself to accomplish....
- Get a gym membership. Go to a gym where your able to track yourself compared to others. Since you have LOTS of time, you should be able to kick ass since most track activity. The longer you're working out equals more 'activity'. You're not doing anything else so wake up at 4am, work out for 3+ hours and set either weight loss, heart rate goals or strength goals. Get pissed off that some 20-something chickee who does aerobics is kicking your ass on points.
- Get a Reddit account or even better use TERB since it's not as heavily moderated. Argue with every dickhead there. In order to effectively argue, you really need to understand the subject. This will create stress but additionally, you will completely learn new subjects. Argue the opposite of what you believe. This will stretch your thinking and will take a LOT of time. Warning though, you'll quickly end up really, really looking for someone to tell you something new...and get bored. Most people aren't that smart & just argue the same old stupid dumb shit (see the political section in TERB). But, it will take a few years if you're lucky.
Good luck.