There is nothing more depressing - and I don't believe I'm being hyperbolic in the slightest with this statement - than thinking of what could have been. It's the center of grief. You don't cry because the person you lost isn't there right that very moment, you cry because they never will be again. You cry because the future you imagined with them in it will never come to be. In my life, in my relationships, I have these fully imagined futures with everyone. Living a very long and happy life with my husband, seeing my siblings find their callings and partners in life. Seeing my parents get old and retire and relax. Seeing my son become the amazing man I know he will be and living the fullest happiest life. I even have a future envisioned with Nana moving over here in a couple of years and spending many happy years with us. She's only 75. I know that's old but she could well be with us another 20 years. Or she could well not wake up tomorrow. If she didn't wake up tomorrow I would be sad I didn't call her more, even though we talk once a week. I would be so sad that I would never see each other again, even though she has been present for most of my life. But the grief would be unbearable for a long time. The grief of knowing my son wouldn't get to grow up with the woman who was such a huge part of my life. The woman who only grew more and more interesting to talk to the older I got. The person who was more interested in my life and everything about it than anyone I knew. A woman who showed me complete and unconditional love. Ever since my son was born, I have imagined him knowing my Nana, now our Nana, as well as I did. I know she would have to leave us when he was young, but I expected him to have as many years with her as I had with Oma, her mother. My great-grandmother. And Oma was a huge part of my life for 21 years. So if Nana didn't wake up tomorrow, I would be sad for me and I would miss her, but what would crush me is the thought of all those visions of the future I had of us together in Valencia and her reading Richard Scary books to my son and walking her dog around the streets of Valencia. Having to come to terms with the fact that those would never exist. That's what would be a knife to the heart.
So... What if I make a conscious choice to try and not envision the future? What if I don't spend my time daydreaming about what life will be like soon or far into the future? Everyone always talks about living in the present, and I have always understood that to mean don't focus on the negatives of the past nor the worries of the future. I had never until now considered that this philosophy of "living in the present" could very well be cautioning the negatives of being too nostalgic or hopeful for the future. Even if they're positive and come from and foster positive emotions, they're still not the present. You've never had this moment before and you never will again. 
Even as I type this, I am staring at a rather annoying scratch on the screen of my ReMarkable where I'm writing this. It's a very pronounced scratch on the screen that casts a shadow and makes itself twice as known in strong light. I look at it constantly and I always wonder what exceptionally careless event led to its creation. It's the only scratch on the whole screen so it must have been something exceptional to be the only one in 6 years of owning this wonderful thing. I imagine that I can go back and see it happening, or better yet stop it from happening. I imagine all the different careless things I could have done to cause it. And suddenly I realize that I have wasted a significant portion of my evening writing time on these thoughts. Imagining the impossible. Not living in the present. And I wonder what could have caused us to evolve to crave walking through the past so strongly. We - or at least I - crave this ability to walk through and relive the past so strongly that we devote sometimes hours of our day to just thinking about being able to do it. It shouldn't provide us with any satisfaction. Thinking about eating a burger doesn't give me any satisfaction whatsovever, so why does thinking about being able to relive these moments - good or bad, significant or fucking pointless - feel like it scratches an itch? Is it good for us? Sure we might make better decisions if we can go back and analyze situations and see what we could have done better, but that application seems like such a tiny sliver of the times that I've had these thoughts – it seems in my case to be insignificant. 
Another possibility is that it's just me. I sure don't think that's the case but I also live with a man who doesn't seem to ever think about the past or present. But I'm pretty sure he's the exceptional case here and not me. The longer we are together the more I see that he is a strange and extraordinary man and I - who spent my adolescent and teen years doing everything I could to be a unique non-conformist -  am pretty basic and can shockingly well be summed up by my Instagram algorithm.