I've just bought my new ticket. I'm staying a whole week longer. I talked it through with the boyfriend™ and he was all for it. He said, and he's right, that now I was staying with him for five weeks and it went by sooo quickly, that I need to stay longer if I can. I'd also feel pretty stupid coming back for a class, that I don't actually have. Soo I booked a new ticket. I found one for 40€ for a pretty good connection so I went ahead and bought it before it becomes more expensive.
I'm going to ask my friends if someone has use for my old ticket. It doesn't have my name on it, so I could theoretically sell it. It's only for this specific date/train, though and they'd need a loyalty card from the railway company, so I guess it's pretty unlikely that someone wants it. If I give it back, they'll only refund me half the price.

The downside of this (uh, yes, there's always one I guess): My best friend is coming to my boyfriend's city, too and we were supposed to go home together. 10+ hours on the train can be veeeery long, so it's always nice to be with someone. We had tickets for the same train already. I feel sort of bad about it, because I was really looking forward to go back with her and I could tell she'd feel more comfortable travelling with someone else