TL;DR

Don't have time to read the whole ebook? Here's the summary.

Dealing with Late Payments is something we all unfortunately face when self-employed - and if not managed well, can negatively impact our mental health.
Remember, providing you have delivered what you promised, you are entitled to be paid on time, and have legal and moral rights to charge a penalty and interest if an invoice is overdue.
You are not responsible for your clients' internal accounting processes or poor business behaviours, so should not feel guilty or awkward asking to be paid on time - it is a business relationship first and foremost, so keep it transactional.
Aim to prevent it from happening upfront with clear communication around payment terms, scopes of work, acceptance criteria, and confirmation of receipt of invoices.
Have a process in place for handling issuing invoices and dealing with payment reminders, and automate it as much as possible to reduce the workload.
Start with positive communication, but be willing to escalate if it isn't working - and remember there is always another option to explore - don't just write it off, and chalk it up to 'experience'.
Debt and financial worry can lead to poorer mental health - don't bury your head in the sand when it comes to tackling money related issues.
Stay aware of what influences your mental health, and don't struggle in silence - build up supportive networks of other freelancers, and seek advice when you need help.

Now, let's dive into the detail...