Welp pls

My bf wants me to be friends with his ex and Ird why he wants me to do that. Me and his ex had a bad start but we talked it out already.. I am trying to wrap my head to this idea but somehow it makes me just wanna cry and feel like I am dealing a break up. It hurts. What should I do man 😔 I dont wanna doubt but I doubt them that they are hidding something behind my back but they keep assuring me that nothing is going on. The ex has a bf too. I am just so scared. I need help.