There's no version of this that isn't painful, and nothing here makes it not painful. The practical part still helps. That's all this is.
The principle
The door stays open. You don't push on it.
When an older child has stopped speaking to you, everything you send should say the same thing:
I'm here. There's nothing you have to do.
Send things with no obligation attached
A photo. "Saw this, thought of you." Nothing with a question mark, and nothing that needs a reply to be complete.
Keep it lighter still than you would for a younger child. Teenagers read pressure faster than anyone alive.
If they reply, and it's angry
That's contact.
Don't defend yourself. Don't correct the record. Don't explain what really happened.
That's fair. I'm sorry it felt like that. I'm here whenever.
This is the hardest part, and the one that matters most. When a child who hasn't spoken to you in months finally says something unfair, and you can prove it's unfair, the pull to correct it is enormous. Don't. You'll be right, and it will be the last message you get.
Never ask them to choose, relay or explain
Nothing about the other parent, ever. They already know they're in the middle. Don't confirm it.
Why restraint
At this age, a child's own wishes carry real weight. Restraint isn't only kinder; it's the one approach that can't be used against you.
Being believed isn't the goal. Being reachable is. They turn out to be very different things, and only one of them is in your control.
If you're struggling with this, and many parents do, a counsellor or a support organisation for separated parents can help. You don't have to carry it alone.
General information, not legal advice. Court process differs between countries and regions; check anything that matters to your case with a qualified lawyer where you live.
Chapter 21 of the workbook covers indirect contact, with a section on older children. Worksheet N, the indirect contact log, keeps a dated record of everything you send and whether anything comes back.
See the workbook & spreadsheet