Just popping back to say that I've now carefully read every reply. I didn't get home until 10pm on Monday after a long, arduous journey (the taxi from her house to the tiny rural train station cost €60 
) and needed yesterday to switch off.
@ILoveMaxiBondi really does have it spot on, as does almost everybody else. It's funny how you can be the last person to see the things that are so obvious. When I posted this, I wasn't sure what to expect but it certainly wasn't such astute observations and advice.
It's true that I don't value myself, or my time. On reflection, I went into that situation with a sense that I was a guest indebted to her (not sure where that comes from), and was desperate to please her in order to make her feel having me there was 'worthwhile'. I fear rejection, so at each point (and I was aware of this), I was afraid she would tell me it 'wasn't working out'.
I knew that it wasn't right fairly soon when I was having to try to find the least rotten-looking vegetable to cook with, or that the meat I was expected to make last four days was only enough for two normal-sized portions (this was a British ex-pat in western Europe, not a third world country FWIW) and I considered walking then. I even looked at flights. What stopped me was a a concern that I was expecting too much, a fear of confrontation and of being told I was useless, as well as me giving her the benefit of the doubt that it would get better and perhaps that she was struggling for money. (But as my friend said, if she could afford to buy pot, she could afford to buy food.)
I see now it was exactly the same in my abusive relationships. I thought that I was expecting too much not to be treated so badly, and I forgave and forgave on the basis that they were struggling for xyz, and eventually because I thought I deserved it. In the last 18 months or so, however, I've walked away from two men at the first hint of trouble. So I'm getting there at least. 
Somebody said something about being careful who I let into my intimate space. This is something I've worked on a lot over the past 18 months or so, but let slip with this woman. We were swapping stories about our childhoods and ex partners and I mentioned a little bit about mine - something that these days, I normally only divulge to people I am close with and trust. So she knew I was vulnerable.
At home, I work in a highly-skilled role but I agonise over hourly/project rates and always end up selling myself short for fear of rejection, or a client telling me I haven't done enough for the price they've paid. I know I am skilled yet I feel I am worthless. I'm now realising this seeps into all areas of my life and it's holding me back.
I intend to do this:
I think you need to plan your life on more standard terms. You work? You get paid. You're in a relationship? You feel taken care of.
Thanks Mumsnetters. I'm sorry it sounds clichéd but I mean it sincerely... You have genuinely given me the slap, tools and motivation to start turning my life around.