How to replace a battle?

On paper, everything in my life is what I would ask for but its not making me happy. Im a 38 year old single male and currently my mood is very low. Im easily confused, tired, weak, totally lacking energy, enthusiasm and although I dont think im at risk of suicide I do find myself asking "what is the point of it all?"

The explanation for feeling like this quite straightforward and comes in three parts. There is a "midlife crisis" element to it. I dont really want or need for anything material anymore. There is an element of loneliness and although I have friends and partners they dont seem to appreciate the things I appreciate and I dont appreciate what they appreciate. This does nothing to convince me that I should share my life with anyone. The biggest part of the way I feel, however, is that this is the first time in my entire life, including childhood, that I have not been battling something. Although that should be a good thing I now realise that it was those battles that were keeping me motivated.

So what do I do? I look around at how other people spend their lives and it all seems so trivial. There's no challenge to any of it, for me. So im left with two choices. Either sit back and try to be like everyone else who is failing to convince me that they are "enjoying" life or I can replace the long grueling challenges that I no longer have. But with what?

I realise i'm very lucky and i feel bad that I sound so ungrateful, but life isnt "doing it" for me anymore.