I totally get it. I tend to have the same responses as you, and I'm not really sure how to give you advice! (Because I typically just do what the other person wants too).

Does she have any idea as to why you were off work for two weeks? It was 2 weeks medical leave - so why not just tell her that due to that, you're just wanting to get slowly back in to things and want reduced hours so that you don't overwork yourself and need another leave? Is there any chance that you could tell her that you're feeling stressed out at the idea of coming back and really just need to take things slowly? (ha look at me being a hypocrite, but it's what I'd be telling myself to do too - although I'm not sure that I would do it!)