[Rivière]

It sounds like your boyfriend wants to have his cake and eat it, so to speak. In regards to telling him to make a choice, no don't do that. That's not fair. You can't force a person to make a choice, it must be done on their own.

I'd also like to point out that you seem very concerned for how well you're treating him, including fears that you're coming across as toxic to him, yet have you ever thought of how he's coming across to you. Judging by what you've written, he's the one that's toxic, not you.

When he asked on another forum if he should split with his girlfriend and they said yes, he got upset because it's evidently not the answer he was looking for. He wanted people to tell him, 'Yes you can continue to have your cake and eat it', except, they didn't. His behaviour isn't fair on you. He clearly doesn't care about how you feel in this situation, just that he wants both a boyfriend and a girlfriend, without any regard to the well-being of those he's with.

Adding on to all of that, while you've mentioned his girlfriend quite unpleasant to deal with and treats him poorly, from an outsiders perspective, I would say there's a reason for her behaviour too. For all you know, she could be jealous that she's not getting his full attention because she has to share him with you, just like how you have to share him with her. She may be feeling pushed aside and/or only agree to be in an open relationship with him because she doesn't want to lose him, so has settled for sharing him so she can at least have him around to some degree. We could spend ages speculating, but there's always more to a story than what one person says. Bear in mind your view of his girlfriend is based off what he has told you. Of course you're going to take his side because you obviously care deeply for him, but getting her side of the story wouldn't hurt.

If I were in that situation, I would leave him. He clearly doesn't care about you in the way that you care about him. Sure you'd forgot' he had a girlfriend, but based off what you've said, he'd made heavy implications he wasn't with her anymore. Had you known, you would never have agreed to enter a relationship with him.

Ultimately, the decision is yours, but you deserve better than to be with someone who only gives you half their attention, and treats you like a second-rate partner.