Simmering lemons for hours, leaves them quite bitter?

I have a lot of lemons. I drink a lot of tea.

I’ve been experimenting, by slicing lemons, putting them in water, and letting them simmer for a long time, reducing the water 2-3X, then impulse blend them, and put them in a tea cup with some simple syrup.

The first few times I did this, the taste was great.

Lately, the taste has been completely bitter. The lemons are fine for other "normal" lemony things.

I have tried cutting off all the peeling, and just have the inner part, and that was bitter as well.

I’m stuck. My only remaining idea is that the seeds might be bitter, causing the problem (maybe my earlier lemons were seedless? Is there such a thing?)

TLDR: I’d like to simmer lemons for several hours, impulse blend them, add some sugar and have a tasty hot drink. Instead it ends up totally bitter. Why? Is there something I can do to remedy this?