Chasing money is awkward. You don't want to sound sharp with a customer who might send more work. You also don't want to fund their delay with your own cash. So the reminder waits, and the longer it waits the harder it is to send.
None of this means you're careless. It means the chasing was never given a proper place in the week.
What you can do this week
Open your bookkeeping tool, or a simple sheet if that's what you use, and make a list of everything still unpaid. Put the oldest at the top. You don't need a perfect system. You need to see them in one place, including the date, the amount, and whether you've already sent a reminder.
Pick one morning, the same one each week, and look at that list for fifteen minutes. Send the reminders that are due. If a reminder still isn't written, write a short one now and keep it for next time. Protect that slot the way you would protect a meeting with a customer.
Agree the tone with yourself in advance, so you aren't inventing it while you're annoyed. A first note can be plain and polite: the invoice number, the date, a copy attached, and a request to confirm when it will be paid. A later note can be firmer, and still civil. Write both while you're calm, and reuse them.
Send the reminder from the same place you send the invoice, so the thread is easy to follow. And once a payment lands, mark it the same day, so the list is something you can trust.
A useful extra is a line on the invoice itself that says when you will follow up, in plain language. People then know the reminder is part of how you work, not a personal prod.
If you'd rather not carry this yourself
We'd set up a simple follow-up sequence in the bookkeeping software you already use, built around how you already invoice, not dropped in as a package from outside. Overdue invoices would be spotted, and a reminder drafted. If it's still unpaid, a firmer one would be drafted next.
Nothing would be sent without your say-so. You'd approve the wording before the first one goes out, and you can change it whenever you like. After that, the drafts come to you to check, or they go out in the form you've already agreed, according to what you prefer.
The software stays in your name, on your card. You own the account and you pay that bill. We charge for the work of setting it up, and for looking after it if you want us to. If we step back, the reminders keep living in your system. Nothing has to be moved.
It won't make every customer pay on the day. Some people need a prompt, and some need a few. The point is that the prompting no longer depends on you remembering, late on a Sunday, that you still haven't asked.