I've found that the more I think about it the worse it gets - which makes sense: it's habit-forming. Try this:
1. Stay on the SSRIs - they can help you learn how to break the habit.
2. Pick a "safe topic." It doesn't have to be interesting (although that probably helps), but should just have a good chance of not leading to existential issues.
3. Next time you start thinking about existential issues, start thinking about your topic instead.
I have no idea if it works in the long-run but it's what I am doing right now. I'm not sure if the underlying cause for the anxiety will ever go away, but if I can habitually stop thinking about it then it might as well no longer be there.
Also, it's usually worst when I am trying to go to sleep. I put on YouTube with brain-numbing content face-down next to my pillow: nothing stimulating (interesting or humorous).