[F] Dildo training advice

So I started dating this guy recently that has a really big dick that is way too big for my pussy. So he actually bought me a really big dildo so I could practice and loosen myself up with it so it wouldn't hurt so much when we have sex but today I tried getting the dildo inside me and I could barely get in a couple inches in before it started to hurt too bad and I stopped. In a weird way I like the challenge of being able to take his cock inside me without a problem but I realized how much practice I need and I'm afraid of the pain :( I guess I just was hoping to get some advice or encouragement on how to stretch myself out for him in the least painful way possible. He also knows I've done anal with other guys and wants me to do it with him but I don't think there's any way I could prepare my ass for what would happen if he put his dick inside me :/