This happened to me when I was 18 and here’s my advice:

You need to document EVERYTHING. Try to keep actual messages, but if you can’t then you MUST do screenshots with time stamps.

File a police report. They will warn your ex to leave you alone. That way if your ex contacts you again, you can go back and escalate your situation. You’ll have a basis to request something more serious like a restraining order.

Tell someone you trust.

Change your phone number. I did this and my ex somehow still found it, but it made it much easier for me to ignore him because he never got confirmation that it was my correct number.

I know this seems dramatic and I’m not trying to scare you, but please — do these things now. Don’t wait and see if they try something worse/more extreme.