In my case, it turned out to be undiagnosed ADHD. In fact, "Do you have trouble focusing on conversations you're having?" is one of the regular screening questions psychologists use for it.
When my therapist first suggested I might have ADHD, I actually didn't believe her. It took a bunch of reading about ADHD to change my mind; in hindsight, my understanding of ADHD—based on popular culture—was pretty far from reality. Doing a bit of research and reading about ADHD from the perspective of people who had it (as opposed to the perspective of, say, teachers dealing with ADHD students) really helped. I found Driven to Distraction[1] a particularly good overview, with a lot of insights and stories that resonated with me 100%.
Since getting diagnosed, things have gotten much better. Just knowing about it was a real relief and let me start building habits to deal with it more effectively. Later on, stimulants and meditation have both helped even more.
[1]: https://smile.amazon.com/Driven-Distraction-Revised-Recogniz...
I've been using Headspace[1] as an introduction to meditation and it only took a few weeks of regular meditation, working up from 5 minutes a day to 20, for me to feel a real difference. I didn't know what to expect going in—I was a bit skeptical of the whole idea, to be honest—but I'm glad I stuck with it.