I love to get immersed into my characters, but it seems that when I play a thief, I start a new warrior playthrough. When I play a warrior, I want to play as a mage. When I play as a mage, I feel like making some alchemy build.

Natrually I end up with a dozen playthroughs at once, which can ruin immersion for me personally.

How about you guys? Do you force yourself to stick to one playthrough? Or are you the same as me?

I stick to one. Usually though, I'm already plannijg a future character.

For me, I can only have one playthrough at a time.

Even thinking about past/future characters ruins the immersion for me, and once I begin to do so, I either take a break or simply start anew.

I have sever characters, IMO it does not affect my immersion one bit. I also have a severe case of Alt-itis.

I only ever play as one and stick with it until I retire them. Sounds like you have a case of restartitis!