I thought you guys would like to hear how this story ended.
A few days ago I told him that I'll need more distance from now on because I couldn't handle the situation any longer. I tried to make him understand that I don't want to blame him but that I've tried all I could, but now think it's time to protect myself.
Unfortunetaly he didn't take it very well. After first begging me to stay, he started attacking me verbally, when he realized that I was serious. He said he hates me for being willing to break his heart and throwing it all away over not getting a stupid little title that virtually means nothing in reality. That he has shared more of himself with me than he will ever share with her or anyone else, and how stupid it is of me not being able to see it. I tried to tell him that although I believe him how much I mean to him, emotions are by definition not logical and that there's nothing I can do to change it. He called me an ungrateful asshole for not being able to appreciate that he has given me all he could
I asked him why he gladly makes her his girlfriend, when she says she wouldn't be able to stay otherwise, but when I express my needs, that are quite similar to hers (or actually even a little less) he gets upset. I can only guess, but I think it is mainly because he's pissed off that his plan to keep both off us close by making her his girlfriend didn't work out, and of course, it is easy to blame the one who doesn't want to play the role he chose for them. I guess he missed to take into consideration that I might have needs too, because to him it was logical that this is not a situation that someone couldn't handle. Though from my experience, I think most people would feel hurt in my position and couldn't come to terms with the situation.
He then texted me that without our relationship he thinks he really doesn't want to go on living, and then deleted my number afterwards. I haven't heard from him since, and I don't expect him to show up again. I mean, I knew he would feel hurt and react very emotionally, but I thought this "I go kill myself and it's your fault" thing would only happen to other people... Luckily, he didn't succeed to make me feel guilty.
I wonder how she would feel about it all if she knew it. Not only the physical cheating. I think he's cheating on her emotionally as well. I think, people who want a monogamous relationship usually wouldn't be happy to hear that being their girlfriend/boyfriend means "virtually nothing" to their partner. As well as knowing that their partner has a closer relationship and shares more of himself with someone else. They want to come first, and be the most important person in their partner's life. I think he will keep her in the dark about it though. I haven't met her, I couldn't. I don't know if I should try to contact her somehow and talk to her about all this, but I'm not sure if this is my job, nor if it would lead to anything. I could imagine that she would side with him, resp. the version of the story he's selling her.
I feel ok now. I didn't feel that my needs were met in this relationship, I wasn't able to push my limits, and therefore went away, so it's fine. I'm not blaming anyone for it, so I'm pretty much at peace with the situation. Of course, it is sad that it had to end in this very unpleasant way, and I miss the good things we've shared, and there is an empty space in my heart now. But it's ok.
Thank you for your support and your opinions. It helped me a lot to find my own position, and especially to accept that I have limits and needs, and that it's ok to have them and stand up for them.