Advice based on USA style system.
Do not pay it. Do not talk to the creditor.
What you have is old debt. The debt is not still accumulating, it has stopped some years ago, and now, the debt is slowly aging out.
After 7 years, it falls off your credit report.
After some shorter amount of time, 2-6 years typically, it will become "time-barred" because it is beyond the statute of limitations for lawsuits in your state.
If you pay on, or acknowledge the debt, the clocks restart.
So don't flipping do that.
"Acknowledge" typically means in writing. Paying on a debt is the sincerest way to acknowledge a debt. Which is why we are not doing that.
Paying the debt fully.... restarts the clock and gives you 7 years of bad luck on your credit report.
Wait, what? Paying on a debt makes my credit worse, and not paying on it makes my credit better!?? What madness is this? The credit industry's madness. You signed onto this when you chose to do business with them by cosigning. It's not moral... but their house, their rules.
You have insolvency armor
Since you don't have any money (so you say), they probably will not sue you. Generally it is not worth suing over an amount below $5000-10,000 because if the "debtor" chooses to put up a smart fight, their legal expenses can be more than they can ever hope to recover. But that assumes a solvent opponent, if you're also broke, then they can't get blood from a stone, and any legal expenses are a total loss.
More likely than not, they are getting "antsy" because they can read a calendar and can see the Statute of Limitations is about to expire. They're getting more and more desperate to get you to "acknowledge the debt" so they can restart the clock. And this works. Citizens are finance-stupid to begin with, and can easily be whipped into a blind panic where they will do anything if they think that'll make the phone stop ringing.
You have all the power here, don't blow it.
But... The debt!
Again, their house, their rules.
Anyway, it's not like they're only calling you. They're calling him, too, because he is the main debtor. So forget any thought of suing your ex-boyfriend, because if these experienced collectors can't get any blood out of that stone, you sure can't! A lawsuit from you on him would just muddy the water, and worse, would acknowledge the debt, which you know definitely not to do.
Ah, right... do something.
Yes, there's always that urge to Do Something, isn't there? It's like an itch. They are inflaming the itch with their constant calls and mails, that is the point, to get you to do something, anything, on the hopes that you do something stupid from your ignorance of the system. This works. This makes bank. That's why they do it.
Well, you can tell them to Stop Calling. Under FCRA and similar law in most developed countries, they have to stop phoning you after you tell them not to. Don't you dare state anything about the debt. That's "doing something".
The plain mail letters, throw them in the trash. That's "doing something".
But really, I'm gonna frame challenge and say that your precondition of "what do I DO" is wrong, because 99.9% of this is resisting the urge to do something. Resisting is doing something.
Here's something you can positively do. Let It Go.
Do the Right Thing
The moral thing is to pay your debts as agreed, when agreed. Time is of the essence. Once you blow it, you can't use money to go un-blow it. You can't un-ring that bell, that's a conceit.
The system as-designed doesn't even want you to try. There is simply no moral way to pay money back to a company this late in the game. Doing so does great harm to you because of a side effect, and that new harm is not moral. I call this "insane" because I don't have time to write a 200,000 word book explaining all the factors and how this has been carefully honed for 400 years amongst competing interests, most of whom are in fact deeply religious. But it has.
It is a conceit that your own morality trumps all: trumps laws of the land, trumps your agreements with others, and most of all, trumps the other person's morality. That isn't morality, that's hubris. In an agreement, everone gets a say on what is moral. The industry has a standard for that.
What they want is very rational. Your original creditor wants this incident behind him. And behind you. Forgiveness isn't just Decent, it's good for business. They do not want you mired in the weight of debt for decades, because people in bankruptcy don't buy new cars or go to the theater. That is why the cutoff is 2-7 years. They want you to learn the lessons, get your feet back under you, and come back as a good customer doing profitable business with them.
And that is exactly what you should do. Remember them and select them preferentially when you are doing well.
This is rational.