Well I noticed my NAT went from open to strict very suddenly and wouldn't change back, so I decided to give port forwarding a go. I've done all of the ports following this guide, but the TCP Port 80 says another service is already doing that. I did everything else and my NAT changed from strict to moderate and I could get into games, but would it go to open if I managed to work out how to open this port? If so, how do I do it? Thanks.

It would most likely stay where it is at even if you opened port 80. This could be an issue with your modem or router. What internet service do you have? Did you verify with your internet provider that they didn't change anything in the modem? I have seen an issue where a new modem had some bad firmware that caused nat to show up as strict.