Hello @chubbychipmonk - you sound in a very similar position to me. I was diagnosed in August. I'm a bit older than you - 49, but also otherwise fit and healthy. I found out straight away that my tumour was ER+, but also had to wait for the HER results to come back before I had a confirmed plan, as I would have had chemo first if it had been positive (it was negative). Mine was Grade 3 too, and I also felt like I'd been given a death sentence, and it was a huge shock as everyone who had felt or examined my lump said that it was low suspicion/not of immediate concern. I also knew it was in at least one lymph node at diagnosis (it's much better if it isn't).
I had surgery in September, which I found quite easy to recover from, and am mid way through chemo. Chemo does feel like a long haul, but I am trying to stay positive, and I don't actually feel all that bad most days - I've certainly felt poorlier at other points in my life (for me, it's not as bad as Covid, or as bad as recovering from a caesarean with a sleepless newborn). I've been out to shop for a new sofa and for a treaty lunch today, for example. So don't despair if you do need chemo.
The fact it's Grade 3 obviously makes the risk of spread higher, but I did read somewhere that there is evidence that higher grade tumours can respond better to chemo, as chemo targets cells that are actively dividing, and the rate of division is higher in a higher grade. I also find it helpful to know that Grade 1,2 and 3 aren't actually 3 completely different entities - there are many, many different types of tumour at a molecular level, and the grades are just one way of putting them into groups - and an old-fashioned way at that, which predates genomic sequencing. They are just statistically determined cut off points on a scale - or actually 3 scales which look at 3 different characteristics of the cells. I might not have explained that very well, but somehow that made it feel less scary to me.
I'm not trying to minimise the fear - I was very frightened at the beginning, and still am sometimes. But I have found that just taking things one step at a time helps - and as someone said upthread 'don't borrow tomorrow's sorrows'. (I hadn't heard that phrase before, and like it a lot.) Please keep posting and asking questions here if it helps. Sending positive thoughts.