What to do about teen daughter having a breakdown after being told about my pregnancy?

My husband and I have been married for 6 years and we each brought a child into the marriage...me a daughter that was 7 at the time and him a son that was 9. We just found out I'm expecting which we are both excited about. Our son is even ecstatic about it. However, my daughter had a complete breakdown last night telling me that she's never had to share me with anyone and now she's going to have to share me with "THAT BABY". She kept saying "THAT BABY" this and "THAT BABY" that and she knows things are going to change because "THAT BABY" is going to have to go everywhere with me and her and she won't be able to have her time with me alone. I have tried numerous times over the past couple of days to make her understand that things aren't going change besides...she's about to be a sopre in high school and I'm sure she'll be hanging out somewhere with her friends here pretty soon. I feel bad because I had no clue that she had those type of selfish tendencies about her. I was totally shocked when she was expressing herself the way that she was. Has anyone ever experienced this and if so, did things get better after the baby arrived? My daughter was the 1st child, grandchild, niece...the first everything. Any encouraging uplifting advise would be helpful. Thanks!!!