Hi Lee. Could it be infected? I'd see a GP. Natty had a rash on her chest and on her bottom for months that nothing would get rid of. (She has pretty sensitive skin anyway, so like you I just kept trying different things.)
Anyway, she ended up needing a course of antibiotics for something else and it, along with a topical cream cleared up both rashes. The cream we used was called Advanten which we only had to use for about 4 days, and both rashes were completely gone. I'd be careful with this stuff obviously, as who knows what scary magical chemicals are in there! But when you've tried everything, sometimes there's no alternative.
Personally, I'd go see the doc and see what they have to say. GL with getting rid of it.
And OMG, you are not a bad mummy at all!!!