I don't know if it's just me but whenever I've gotten super invested into a drama with a specific coupling, I have a hard time watch a new drama with one of the actor/actresses as the leads. Example: something in the rain, I was super invested in the pairing and couldn't watch One Spring Night but had no trouble watching CLOY. Etc. What are some dramas this has happened to you for (if at all)?

I actually watched the same kdramas as you, but i watched one spring night first. Since i loved the ML more than FL, i didnt have a problem watching Something in the rain(my fav kdrama couple). I watched a few kdramas in between something in the rain and CLOY to make it easier for me to enjoy CLOY.

I don't have that issue. I get invested in individual actors rather than pairings and if I do get invested in someone I search for their other works and watch them as soon as possible. 

No, I don't have this problem. If I get invested in an OTP, it's because the ACTORS have done a good job and/or had good onscreen chemistry. So I'll look out for those actors again, to see if their skill at acting extends beyond one specific Drama. I've never shipped an OTP IRL, but have been pleasantly surprised when it happens anyway, as with Nigeru Wa Haji and Le Coup de Foudre. Knowing those actors are IRL partners would not stop me watching them act with others, as I know they're good at their job, which is pretending.

I am more interested in actresses than their characters and pairings. If some actress becomes my favourite then I like to watch her other roles. If those other characters of her are happy in the end, it doesn't matter much to me, who they are paired with. :)

I kinda feel like I fall in love with the characters played not the actor themselves. So to me, it's like meeting a doppelganger as I watch their other shows.

No.

I find nothing weird with actors starring in new series or movies. I actually like their character changes between shows, sometimes I can't believe it's the same person. But it's not at all hard, I'd describe it as fun.