Hi Amy, welcome to this site where I know from experience you will receive lots of support. We have all either been where you are now, or currently there and whilst no two stories are the same we all have one thing in common and that is we have been diagnosed with BC.
I am sorry to hear it has come into your life at an early age, I was 49 when I was diagnosed and underwent a mastectomy, chemo and radiotherapy. Don't ever beat yourself up for not getting something looked at sooner. We all tend to do that but you can't change the current situation so you just have to move forward and work with your medical team to get the best possible outcome.
A mastectomy affects everyone differently. For me personally it wasn't as bad as I thought it would be and I chose not to have reconstruction. That is also a very personal choice.
The best advice I can give you is to remember each persons journey is different. You have to do what is right for you and what you are comfortable with. I wish you all the best with your upcoming surgery and am always here if you need to talk, vent, ask questions
Warm regards xx