So, I've just reminded myself that I'm still 'facebook friends' with a person who has chosen not to pay me for the work I've done for him (though he's happy enough with the work to use it on his website).
As you can hear from the tone, I'm not QUITE over it yet, but I've been having a think, and I have managed to find some things in this situation that bring me benefit, so without denying that I'm really unhappy that this is what he's decided to do, and quite frustrated at not having any control over the situation (he ignores my emails and facebook messages, and my attempts to call him at his place of work resulted in that organisation's automated system saying there was nobody of that name working there...). Here's what this has done for me, though.

[little warning here - if I were you, I'd stop reading now. This is entirely for my benefit. This is me standing on a rock, having a howl. Have a nice day and let's speak tomorrow]
1. It's the first time. In seven years of freelancing (three and a half of those full time), this is the first time anyone has decided not to pay me. It came close once before, but the person came round (and, no doubt out of guilt, ended up paying a bit more than the amount I invoiced him for, without ever mentioning it).
2. It's for a relatively small amount - about £200. It's the kind of amount that still makes a difference. I had to choose this week between buying a second hand bike and going to a weekend solstice ceremony. That's the kind of difference it makes - not homelessness, bankruptcy, dreadful situation difference. A bit of a difference. I'm very lucky that it's not more than that, and that it's come at a time where I'm not hand to mouth in the way I have been.
3. My instincts are good. I need to respect them more. There was an element of hard work about our interactions, despite the best intentions. Had I listened to the little voice tugging on my consciousness sooner, I'd have wrapped up and invoiced sooner. Also, when the money didn't arrive and only little bits of work trickled in, I could have stopped right then. I noticed the pattern had changed and felt something coming... and I carried on being polite and nice and helpful, and turning things around at the drop of a hat for him. Instincts are there for a reason.
4. I had my part to play. I made it easy for him. He offered to pay me up front (for piece work that would then add up to the amount he'd paid me). I thought that would be weird, so I said no and did a few months of work for him.
5. Integrity is really, really important to me. It's good to know this and trust me, much as it may sound like it, it's not in a 'holier than thou' way. I have messed up on this front. In the past (one very memorable time when I was 13, involving not paying a good person some money back because I thought I could get away with it. I was then forced to, thank goodness, and worried about that act for years) and, thanks to this unpaid invoice situation, I got to examine where I might be cheating now. Turns out, I was refusing to pay Three for a shoddy service and objecting to their business methods... but I had used some of their service for one bit of what I was refusing to pay for. Crappy service, and they make it hard and costly for you to cancel an account. It's still stealing, though. Now paid. And how about those French coaching lessons, which I had always said to the lady I'd send her invoices for, but hadn't got round to. It wasn't an act committed, but it was a thought that existed. This situation has helped with that. It's really important to me and I feel pain when I go against it. So no more of that. And just to make it clear (to myself, really) - I am not refusing to pay someone and using their work. But it's a matter of planting a seed, and the seeds I want to plant are those of abundance and honesty... so I'll get out a trowel and the Baby Bio and get started.
6. This is the exception, not the rule. That can apply to other areas of my life too. I've made some questionable calls about who it's okay to trust. This one person turned out not to be trustworthy in this particular context. Much as my mind goes to all kinds of silly places, I'm sure we're not dealing with a conman or a sociopath. I'm sure he loves his kids and his wife, respects his friends, is honourable and entertaining. He is friends with people I respect, and imagine to have good judgement... I suspect it's just opportunism. What's she going to do? She's in another country. Nobody can make me! And he's right. Nobody can. The point is (back to point 1), it's rare and (point 3) I have a sensor. I can trust more people. It will all turn out just fine and even when things don't go to plan, it's not actually the end of the world.
7. 'It's not fair' hurts me more than not receiving the money. As in, my reaction, and my ego's indignation - that's what hurts. Him not paying in itself doesn't cause pain (as above - this has come at a good time, when there's been other work - it's not like I'm going to not eat, or get evicted because of this). It's the fact that I'm focusing on what SHOULD happen, that he's using the work, that he's carrying tweeting jolly and/or motivational posts about treating people right, and Being Funny With His Friends - how dare he! Ha. I can choose to hold onto this, or I can choose to let it go. He's unlikely to pay me now, and holding on tight isn't going to change that. He's not in this country, so there's absolutely no point even thinking about making it official. So which am I going to choose?
There's more, but it's late. Thank you. This has genuinely helped. And here we are, salt and pepper. This is how gratitude and a situation that I don't want to be happening can live peacefully alongside each other. And what a mild version of a difficult situation. Hell yeah. Let's go with that!