My boyfriend says he loves me but still sees his ex for sex
Trust is core to any successful relationship, and if you can’t believe your partner then you need to think long and hard about continuing in the relationship.
Though the relationship is relatively new his deception has hurt you deeply. Did his previous relationship really end? Is he playing with both of you? However his ex-girlfriend is not the focus of the problem. It is his behaviour. He is the one making the choices.

