I'm kinda there with the OP. I turned 40 last fall and I've been stuck in a "mid life rut" for a while, as well. Like him, I have a hard time getting excited for a lot of things, and I just kinda go through life pretty unimpressed. And I feel bad about this, I feel like I should be more appreciative, have more fun, maybe do more things. I have a decent job but I've gone as far as I can with it, there is no more upward mobility. It's become boring and is not challenging anymore and I generally despise it, but the money is good. I go through the motions. I have a good marriage of 17 years and I adore my wife, but between work, kids, and our "hobbies" (my band, her yoga instruction), we don't see each other a ton. When we do, there isn't always a lot to say beyond what we did during our day or what we might have to do with the kids. I used to have snowmobiles, dirtbikes, motorcycles, and other toys, but they are all gone and I have no desire to get others. I did buy myself a cool car thinking it would help, but it was more of a hassle and I traded it in on a new truck which I could use a lot more. I do go to the gym periodically and I enjoy it, but I lack the motivation to go more. I used to go every day. Basically, I totally "get" what the OP is saying....one day just kinda blends with the next, nothing remarkable. And I feel guilty because I know it's basically my own fault and nobody else's.
Here's what I figure is true, at least in my case. In my 20's and 30's I was always building, always advancing, always growing. Bigger, better job. Bigger, better house. Bigger, better car. One kid. Two kids. More security and more money. I started at the bottom and was continually working my way up to achieve what I (mistakenly) thought were my dreams, or at least what I was "supposed" to do in life. When the kids were younger there was a lot of time spent with them, doing things for them, or just being with them. Coaching soccer, baseball, trips to dance class, chorus performances at school. Now they are older and they don't look for Dad as much, which makes me kinda sad. I used to have friends, but lost touch with most of them. Most of my friends got married, like me, but then got divorced a few years later and went out trying to have a "start over" and re-live their teenage years, which wasn't for me. Over the last 15 years I've spent so much of my life being and doing what was "expected" that I don't think I really focused much on what I wanted. A few years ago I got back into music and started working again, and it has been great, but even that is starting to grind on me a little bit and I question my dedication. In looking at my life, I certainly have a pretty good one, and I should be happy, but it really didn't turn out the way I expected. Or at least how I thought I expected it to. As a result, I'm looking at my life and realizing that, at 40, I have no idea what I'm going to do with the rest of it. If you had asked me when I was 25 what I would be like at 40, I'd tell you that I would have had it all figured out by then. But I don't, and to be honest it's pretty intimidating. And with my kids getting bigger, I have more free time but don't know what to do with it. Then there's the whole thing of looking at your age; I'm 40. I've been here four decades. But it doesn't seem like it. And it's shocking to realize that in the short time I "feel" like I've been here, I probably have lived half of my life or more. None of that is overly exciting, in fact it's actually quite disconcerting.
I'm hoping to change things up a bit and enjoy my 40's more than I have so far.