You're caught by forest spirit who uses you, feeds on your vital energy and erases your memory every day. Now you don't even remember who you are, waking up on him again. He's ready for a new round! That's the plot.
...

He turned you into a swan, and now he comes every night to ask if you marry him. Patient, charming, inevitable - he will make you his, no matter how many centuries that would take.
SFW slow burn + NSFW after marriage...