My ex tells me too much?

I finished with my bf a month ago because he was still not over his ex gf. We talked things through and I was strong enough to stop our relationship but it really hurt. We carried on seeing a lot of each other and then his ex gf got in touch because her new relationship broke down. He is now really happy because despite the fact that she treated him badly and dumped him, she now wants him back (she hates being single). He asked my advice and I said he might be making a mistake. He is obsessed with her and now he is seeing her again he hardly contacts me. I feel so used as I was hoping we would get back together. I feel stupid. I have supported him over the last 6months like a fool and now he is off back to her. I should have cut all ties, I know but I still love him and it really hurts. He knows I have a lot of feelings for him but he says I should try and be happy for him. How can I come to terms with these feelings of anger and betrayal. Please help : ((((( x