@MrsPP
Losing a breast means different things to different people - and all those reactions are perfectly valid. Your GP may have meant well, and may consider losing a breast doesn’t compare to losing a leg or an arm, but it’s not her breast and her opinion isn’t yours!
A practical course is to consider what you want and are able to do. Do you want immediate reconstruction? There are groups in the network that can advise on reconstruction options (and costs) also on staying flat ( I had a single mastectomy and no reconstruction). You can of course wait and make a decision later when recovered from surgery and able to gauge how you feel about being mono-boobed.
You can make a list of questions. Do you have a breast care nurse? She may be able to advise also.
Most find the mastectomy itself pretty straightforward and usually not painful. Reconstruction can be more demanding.
Most of all, you want to know that your surgery is successful, the cancer is gone and what treatment is recommended to ensure it does not return.
An occasional scream may be fully warranted - none of this is easy. As @tina_m2 says, it takes time and gentleness with yourself. Best wishes.