You're a bit young for a midlife crisis, but covid has forced it on all of us (I entered mine at 39 and covid just crushed it). And the world has changed a lot; a lot of the future we imagined has really been taken away - that is, it isn't available anymore, not because of something we did wrong, but because the doors have slammed shut. Just for one thing, having $500k in the bank ain't what it used to be. But I get the sense that you aren't really defined by money.
Maybe the "haven't had a serious relationship for many years now" part is what you should work on. Just putting your time and energy into a relationship is good for you. Without a relationship you have no future.
I have no kids. That makes me feel like I may have no future.
Being aware that the thing you're fighting, the dread you're experiencing, is really about your fear of not having a future -- that can at least provide an anchor, to focus you on imagining what you want a future to look like and taking the steps to get to whatever that place is. I think the biggest problem is that we're all constantly obsessed with the present and somewhere along the line we stop trying to evolve into what we want to be, and get mired down in the quotidian. This may be a normal feature of growing older, but it's definitely amplified by the overwhelming flood of bullshit we see every day now, which seems designed from the ground up to prevent us from thinking about our future.
Spend a little time and ask yourself if not having [edit: not being able to visualize] a future is your actual problem, and take it from there.