This goes down to how IPv6 is provided. Most ISPs use DHCPv6-PD, as I mentioned. This is why I asked about your ISP, so we know. I'm with Rogers in Canada and they use DHCPv6-PD. I get a /56 from them, which pfsense can split into as many as 256 /64s. I also have a WAN address that's not from within my /56 prefix. They'd have their customers sharing a /64 for the WAN addresses. However, if you want help, you have to provide some info that we can work with. Just saying you have a /48 doesn't say much. We can only assume they are intelligent enough to use DHCPv6-PD and are not dumb enough to provide a /48 on a single network.
BTW, forget NAT with IPv6. It's a curse that was created only to get around the IPv4 address shortage.