I am terrified about what's just happened.

So basically I bought a one day travelcard on Young Persons (YP) railcard BUT I didn't have a valid young persons railcard and that's because I don't actually own one. I used to but obviously I'm almost 30 so I won't be able to use one anyway. So basically yes, I don't have much money at the moment and so I didn't intend to pay the full fare and buy a valid ticket. I admit this. Now unfortunately, I actually also admitted this to the plain clothes police officer who questioned me. Basically as I got off the train he saw I didn't have a valid ticket then got out the paperwork etc. He then asked me a list of questions, I can't remember, I was quite nervous at this point but wasn't too scared as I thought he would just issue me with the £20 fine and that would be it.

He asks for my details etc. write my name and address down correctly, he then makes that call, writes something down and then more questions – did I deliberately try to avoid paying the full fare? I said yes, I thought honesty would be the best policy here. I later mentioned I didn't have much money in my bank account and had some change so used that to buy this discounted train ticket. He asked when's the last time I saw my railcard and I said I can't remember maybe 2 years ago or something.
Then he read the caution and I signed the form and he said it's going to be sent through to South Eastern rail and you'll hear from them at some point. No penalty fare, I didn't get any paperwork etc. and he kept hold of my ticket.
Now what makes my case worse is that I've done this twice before, once I got reported for not having any ticket because I was rushing to get somewhere, the second time again I had purchased a discounted train ticket and didn't have the card for it. On both occasions they just asked me to pay the £20 fine and that was it. However, both times they also took down my details which means they're on my record...and now this third instance.

I think I'm screwed here.