his he just angry?or serious?

When trust is broken, it will be hard to regain back the pieces of it altogether. Your guy tells you that he gave you his trust because he thought that he's the one you loved UNTIL he found out that you did something wrong that made him distrust you. When he gave you his trust, he WAS serious, seriously in love with you. BUT when you made a mistake that brought your relationship with him in turmoil, he must have felt BETRAYED and he must have felt resentful of what you did. So he clearly stated to you that things will never be the same again between you and him. I could see that his decision of wanting an open relationship with someone else is reasonable (you know why). Always think of the possible negative outcomes in your relationship in the first place before mistakes happen. There could be another way to bring back his trust, but it will take you time. Time will heal wounds when you begin to accept that things turn out whatsoever between you and him. You will be fine, don't you worry.

what was the mistake? simple mistakes can be annoying but they can be forgiven .. but serious mistakes will take a lot longer and you must give him space and give him time. there is a saying if you let someone go and they come back then they are yours if they don't then they never were. can i also add for you to put yourself in his place and reverse situation.. how would you react and how would you feel if he had made the mistake and you were the injured one? it seems to me what he needs is time to consider how much he loves you and how hurt he is to see which he can live with the most.. good luck and i wish you both well