...I think you need to take it slow. ^^;
As someone who once faced the same realization of "OMG...have I ever written a story that WASN'T dark and edgy and full of suffering??" a few years ago, I'd like to assure you that it's not impossible for you to expand your range if you really want to, especially if you already write a wide range of stories to begin with. But it's not gonna happen overnight, and definitely not if you start wracking your brains to see if you can immediately come up with a decent concept for a story in a genre you've never written in before....
Like everyone else has said, (a) you're probably overthinking this a little, and (b) it's not actually a problem if you can't write "light" stories (like...it's really not a problem at all, it's fine). HOWEVER, if you'd still like to explore it in the future, I can tell you how I made the transition:
Like I said, take it slow. Start with 'lighthearted' takes on serious stories; write parodies, romantic dramedies, funhouse horror. Get used to emphasizing positive emotions while sticking with subject matter you're familiar with. Comedy will be your best friend during this step, and if you haven't tried that yet either, you should start.
Find a foothold. When you feel ready to write something with lower stakes, don't set your story in a peaceful mountain valley just yet. Try writing slice-of-life starring people in more dangerous settings and circumstances-- the drama and heavy subject matter is still there, but it's no longer the focus.
For example, I would write characters like...an old war veteran and two undead wraiths, or a group of slaughterhouse workers. ^^ And they'd often talk about the dark and grisly work they did (or used to do) but the actual present-day plots tended to be about things like...going out to get groceries. ^^ Or just hanging out and eating apple pie that someone's grandma sent them. Dark settings, dark characters, but light situations.
...It will probably take you a couple years just to master Step 2, tbh. Honestly, once you're there, you may find you don't even want to leave (you could stop at any point in this progression, and again, it would be totally fine).
But Step 3 is taking everything you've now learned about interpersonal conflicts, and focusing solely on that. Because at that point you'll find it won't matter if your characters are living in a hell world or a perfect utopia; you'll still be able to come up with things to write about them. Characters are the true heart of lighthearted stories; the way to tell a good "cute" story is to not only focus on cute feelings, but also on the people they come from, and the reasons why.
With that said, (and I mean absolutely no offense) this:
...Isn't actually true. It feels true, and that's a large part of the reason why it's hard to tell light stories without feeling like they're pointless, and/or won't have an impact on anyone older than 4. But it's not actually true.
Light stories will always have conflicts in them (that's kinda how stories work) but they don't need dark implications to have depth and meaning. The ones I write are about things like...a parent who wants a closer relationship with their child. Not that their relationship is bad, in fact it's very healthy all things considered, but the parent is starting to feel a little more distant from the kid as they're getting older, and they think maybe the solution is to start trying harder to be part of their life.
^There's a lot of story to tell there, if you're willing to look for it and appreciate it. Maybe some comedic hijinks, a little desperation on the parent's part, a little annoyance on the child's part now that their parent is suddenly all up in their business, maybe some hurt feelings around the climax. But in the end, a resolution that says it's normal for familial relationships to grow and change over time, and that nothing lasts forever, but the key is to accept and appreciate what you have for what it is, and make the good times as good as they can be.
Nothing dark or serious there, but the story still has the potential to be poignant and meaningful and relevant to the lives of the audience. Not all lighthearted fare has to be silly jokes or aimless fluff, you can do stuff like this, too.