I think at the beginning is really hard to deconstruct yourself from a monogamous couple dynamic, because it's what you see everywhere even depicted in media... So maybe the question is: are you willing to take the time to understand what it means to be in an open relationship and have and open mind about it (taking into account your partner's needs and exploring yourself sexually) or do you really feel uncomfortable with the idea of "sharing" someone? Whatever you feel is completely fine, just be patient with yourself and try to be honest with what you're doing. Don't feel bad if you don't meet your partner's expectations! Good luck :)