I'm sorry to hear about your ewe.
I agree with all that's been said. I agree about culling her, sad though it is. (I'm sure I'm not the only one has kept a favourite ewe on after she's appeared to recover from mastitis, and regretted it later, when she doesn't manage to feed her next lot of lambs.)
I also agree that predisposition to mastitis *can* be genetic - but there can be environmental factors at play too. If there are any wounds on the teats, for instance, that can create an easy route in for the bacteria. Wounds can happen if the lambs are suckling too vigorously (which may indicate not a lot of milk for them - but also can happen with inexperienced ewes who don't yet know how to balance the milk supply and the lambs' demands), or by physical damage other than suckling, eg., if there are thistles in the grazing area.
I mention all of that because if you can find an environmental factor, it may mitigate against a black mark on the genetics. (Whereas if promising young ewes from a certain line keep getting mastitis, then I'd be not keeping others from that line.)
You've mentioned not feeding the ewes this year, and that the grass isn't stunning. If there's been a cold snap, her milk supply would have plummetted, and 6 week old thriving lambs would have worked her harder, trying to elicit as much as they've been used to. As she's a first timer, she may not yet know how (hard) and when to kick them off, so allowing them to drag her down, injure the teats or whatever. Might be worth thinking about putting out a feed block another time? So that the ewes can get a bit of extra input if they need it? Not a rocket fuel very high energy one, just a low-level backup type.