Dear Ellen
I understand the fear, and the reason for your melt down - but please try to hang in there until you see your doctor, whom I think will be able to reassure you.
According to the American Cancer Society, and to an article published in the Annals of Oncology 2009, the incidence of breast cancer in women aged between the ages of 20 and 24 is 1.4 per 100,000. This means that it is incredibly rare for women under 24 to develop it, and you have yet to reach twenty!
I am an early retired doctor, who spent a number of years anaesthetising women with breast cancer for primary surgery and for reconstructions. A little under two years ago, I developed it myself, so I probably know a little about it as a disease.
I seem to be fine now. However, you will understand that I have enormous empathy for women of any age who are suffering the fear and anxiety that you presently are. Until you have been properly examined medically, no-one other than your doctor is going to be able to reassure you adequately that whatever your lump is, that it is benign.
However - there are many causes for benign breast lumps, such as trauma (you mention that you work with horses - have you been kicked in the chest in the last year?), benign breast mice (fibrodenomas) and fibroadenosis, although I would think that you may be a little young for that. Breast mice (fibroadenomas) are very common in young women (especially between 15 and 20, as they tend to develop during the first few years following menarche (your first period). They are benign, i.e. non-cancerous lumps that tend to move around a little, at least under the skin (but not usually all over the place!). I found one of these when I was 21, at medical school, and I was terrified! Of course, it was fine. I had it removed, and forgot all about it.
Whatever the cause of your lump, it is really important that you have it examined properly, so I am so pleased that you have made that appointment with your doctor. He/ she may very well want to refer you to a breast surgeon, and possibly for an ultrasound. This is to be expected, so please don't be alarmed if they do.
Try to avoid consulting Dr Google. There is a wealth of good information out there, but also a great deal of harmful, misleading stuff, too. There are two sites that I could highly recommend - the first is Sniffy Wiffy. This is a family business that sells lovely massage and moisturising products, with much of their profit going to charity, but their message and mission is to increase breast and testicular awareness -- so they have a very user friendly description of how to examine your breasts on the home page. You don't have to buy anything!
It would be good if you developed a routine of self examination, based on an accepted technique, and of course the more that you get to know your breasts, the more likely you are to pick up anything that seems abnormal to you in the future.
The other people who I can highly recommend are those at Breast Cancer Care.
I haven't put the web sites in here, because patient.info will delay this being sent to you, while they (very reasonably) verify their authenticity, but BCC is easy to find on Google - it is right at the top.
BCC have one-to-one telephone people, who are highly trained to listen to your fears, and to offer advice. All their operators have to have had breast cancer, have a partner with it, or a genetic predisposition towards it.
I found them to be of enormous help and comfort when I was first diagnosed, and they may be able to give you some reassurance, and certainly some useful information and advice.
Fiinally - although it is absolutely understandable that you are dreadfully scared - the chance of your having breast cancer at this point is very, very small.
As to your final question - 'what if it is cancer?' - that is something to deal with in the unlikely event that you have it. Treatment for breast cancer has improved so much over the last decade, which is great news, but now is not the time to concern yourself with worst scenarios.
I absolutely agree with Matron, below, that it would be sensible to phone the doctor's receptionist, and ask for an appointment today. This is an entirely reasonable request - no-one would want you to suffer like this for another couple of days while waiting to be seen.
Ellen, I shall be thinking of you. I hope that I have managed to make you feel a little calmer. Please let us know how you get on, if you feel like it ...
Sending big hugs through the ether
M xx