Re: romantic mistakes
I believe it's talking about divorce. If you marry the wrong person, you've made a romantic mistake. I suppose it could apply to long-term dating as well. The leap this study is making is saying that you will be happier with your spouse if you think you can never get a divorce than if you continue to believe that you can split if things don't work out. At least, students who were stuck with a photograph liked it more than a photo that they could change.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.