Calm down... you won't be paying a penny to Horizon if you follow the advice. It's a pity you did to come here before you started the whole appeals process as it is probably that this would all have been over and done with by now.
However, we don't have enough detail to see where you screwed up the appeals process. So, the first thing we need to see is the original Notice to Keeper (NtK) you received from Horizon. You should redact your personal details and the PCN number but leave all dates and times showing. Show both sides of the NtK. Also show us the wording you used in your initial appeal and in your POPLA appeal and also show us the complete POPLA decision. You can try and edit that by breaking top the wall of text that POPLA issues their verdict as.
Once you have show us all that, we can move on to what happens next. What you need to know is that the POPLA decision is not binding on you and this will most likely be resolved when Horizon try to take you to court. That is a good thing. The county court is the ultimate dispute resolution service and should not be feared.
The most likely outcome is going to be that the claim is either struck out or discontinued. However, you will have to weather a whole load of useless debt recovery letters which you can safely ignore.