Leaving a job for a new one – what to say

I'm in a sticky situation and I'd really appreciate some advice. I took on a new part time job 3 months ago as a Graphic Designer (at a very low wage), but I still don't have a contract, I find my new employer difficult to work with although her heart is in the right place, and I correct her mistakes constantly so I don't feel as though I'm working for someone I can learn from. I recently had an interview for a different part time job, same hours and pay, and was offered the position. In a normal job, I would have no issue writing my notice and moving forwards – however, my current job isn't "normal". My boss is very personal as it is a small business (just her and her husband) and the office is in her house. I hear all about her family issues, she tries to hug me goodbye when I leave, her 2 year old child likes me enough to name her doll after me, and she keeps saying how much of a lifesaver I am to her. Whether or not this is true, and I'm sure she would find a new employee pretty quickly as design jobs are thin on the ground here, I am finding it difficult to tell her I am moving on as I know this will come as a surprise to her. I did think about keeping both jobs as they are both 16 hours and I am no longer a student, but I don't think this is realistically possible due to travelling and my health condition (I have CFS). I also know that she wouldn't be able to afford freelance rates (no, I would not charge minimum wage as a freelancer because of all the costs involved). So I am really stuck as to what to say without upsetting her. I appreciate this sounds like a problem I am lucky to have, but I just want to end things on a good note.