Is it over?

My girlfriend and I have been together for about 15 months and she moved in about a year ago. She has 2 beautiful girls, 6 and 18 months, and I have 2 boys, 10 and 8. She is unsure what she wants and is talking about moving out but still dating. I have convinced her to stay on 2 other occasions. She says everything happened to fast and she want to back up. Is this possible with everyone involved? I tried to give her everything she wanted and let her know how special she is at least 3 times a week. We talked about marriage and were ever engaged for a while. I'm afraid that is she leaves she want come back and that I will lose the girls. The 6 year old has spent about 5 days with her dad this year and I am the only dad the 18 month old knows.