You are bound to feel scared Whelk, the waiting for these appointments is very hard. Try not to jump the gun at the moment, the main thing is that you are going to the best place to get your problem sorted whatever it may be.
GP's always tend to err on the side of caution with breast lumps, and a high percentage of referrals turn out to be nothing to worry about.
In the worse case scenario, breast cancer is a very treatable disease these days, - I am 3 years post DX. Don't be panicking about your children just now
chances are in your favour that you will be fine.
When I was going through the waiting process a nurse said to me 'deal with what you know and don't worry about the don't knows' and it's good advice. deal with everything a step at a time. The main thing now is to get through your appointment and then see where you go from there.
Have you got a friend/spouse who can go to the clinic with you for support?, it does help to have some company IME, as it can take a little while if they do several procedures.
The tamoxifen thread is a support thread for people with BC and for anyone going through tests for lumps (we have lots of women come on for a bit of support while waiting for appointments, - it can help to have people to chat with) so please do pop over if you think that would help 
Lots of luck with your appointment, if you've got any specific questions about the tests at the clinic, just ask 