I had one when I was 38 (16 years ago) and I felt much as you do, there's a difference between deciding you don't want more DC, and having that decision forced on you IYSWIM. And I did feel sad about it. I was possibly in a slightly easier position than you in that mine was an emergency and I was very ill, and it was relief to start feeling physically better again afterwards.
Obviously you have been having problems of some kind for a hysterectomy to be necessary - and you will feel so much better in that respect afterwards, but feeling sad is normal - its a big thing, and you need to allow yourself time to acknowledge your sadness and your feelings.
As regards the operation itself - mine was done through abdominal surgery on top of another scar, I was in hospital about five days and it was about another three weeks before I felt reasonably human again. I ended up taking three months off work - but my job at the time involved heavy lifting, if you have a sedentary job you'll be able to go back quicker than that.If you are a SAHM to little ones, you'll need to be very careful not to lift children if you can help it, and be careful pushing a buggy. You won't be able to drive for a while either. Even though hopefully yours will be done laparoscopally, it is a big op however they do it so a longish recovery time.
I don't have any experience of HRT - they left one of my ovaries in so I didn't need it - but someone else will probably be able to tell you about that.
I hope it all goes well for you, - I'm happy to answer any specific questions you have if I can, take care 