So... the obvious thing to say from an outside perspective is that this person is exploiting you (they are), they're not likely to be willing to stop easily (they're not) and that the best way to respond is a clean break followed by getting other work. Based on what you've said, though, that's not a viable path for you. You need work, and if you give up on this opportunity, you're not likely to get another. I'm going to assume that you are correct in your premises, and try to work with what you have.
- You need work. Dropping this job and hoping to find another is non-viable for you.
- The current situation (which your employer likes, and for good reason) is also nonviable for you. It's too much work and stress, and not enough money.
Given that, you're going to need to renegotiate. They aren't holding to the current contract, but that's not even the point. The current way of doing things is nonviable for you. In particular, they set you tasks of unbounded scope for a fixed fee and then they feel free to toss on other tasks as freebies alongside. That's pretty much nonviable on the face of it. That setup is going to inevitably lead to them demanding more for less until you hit a nonviable state, and then putting the squeeze on you until you adjust your ideas of what "nonviable" are and then consume all of that space too. That can't be the way you do business. You need to renegotiate.
I'm pretty sure you're going to have to charge per hour. If you can establish an hourly charge that both of you think is workable, then when she throws you additional tasks, all you need to do is get her to tell you what the priority is compared to the other things she has you doing. You can report how many hours you've worked on each thing and ho close it is to completion. You can let her argue with herself about whether the interjection tasks are more or less important than the overall projects she has you on. At that point, the only thing you have to stand firm on is that you get paid for the hours that you work.
For any other way of doing things... We've already seen that she's generally inclined to pile more on when possible. It would leave you having to argue the price of every task she gives you, and it sounds like she's giving you multiple tasks per week. Further, she's almost certainly better at this sort of arguing than you are You don't want to set yourself up for the sort of grinding psychological attrition that would result from that. Hourly is much better for your purposes. Then all you have to do is give her your best guess of how long it will take (specifically an estimate, rather than a promise) and let her make the decisions for herself.
Now, the fact is that she's not going to like this. Why would she? Right now, she's getting an amazing deal on your time, effort, and emotional wellbeing, and she's not going to want to give that up... but you can't afford to keep giving her that. She's highly likely to make the process of renegotiating highly unpleasant for you, in ways that are going to make you want to give up and let her keep exploiting you. That's the sort of thing that people in her position tend to be good at. You're going to have to suck it up and deal on that one.
It's worse than that, though. It's entirely possible that she flat-out won't be willing to work with you under anything like a fair deal. It may be that she pretty much gives you an ultimatum of accepting her continued exploitation or losing the job. At that point... I'm sorry, but you're going to have to walk. At that point your position at the company is not a long-term prospect regardless. She's going to keep leaning on you, and keep making it worse, and keep reaping the benefits until you break and simply can't anymore and they she's going to discard you, and you'll be in the exact same situation, except that you'll be older and more psychologically damaged and probably poorer. If she cannot be convinced to pay you a decent amount (whatever that means for you) then you must leave, and you have to be aware of that, and have accepted that, before you go into that renegotiation meeting. This isn't a matter of trying to convince your boss to give you a raise. This is a case where you've found that you're in an untenable situation, and you can't stay in it, but you're hoping to work out with her a way to get out of it without having to walk. If she won't give you one, then walking is your only remaining viable option.
