Op, as a single parent I can relate. The amount of time and energy you have to put into starting up and maintaining a relationship, especially with someone who has no dependents, can be immense and end up feeling like just one more load on you when you already have so much on your plate. I feel like this at times as well, I don't have so much time to offer although the money thing is not so much of an issue for me.
What have you got to lose though, by being honest with him? If it's going to end anyway, just tell him the truth: I'm often knackered, run down and ill with how much I have to do, and I'm worried I can't be the girlfriend you want me to be, so that's why I'm ending it. See what he has to say. I would worry less about the money, as my experience is that some men are happy with paying more if they understand they are wealthier, and in the future, if you have a good job, you will also be able to contribute.
You sound down and overwhelmed and getting a kicking on MN won't help your mental state, so I'd get off here, perhaps try to have an easy weekend, see if you can make time to see this guy and tell him the truth and why you feel you can't go forward and see what he says, he may surprise you by being more flexible/happy to take what you can offer, or it may be that it's all too much right now and you need to simplify things.
I do get you though.