M. - I'm not a counselor, but I do work with kids (Jr. High and HS) with my local church. I also suffered with some of the problems that your daughter is exhibiting when I was a little older though... low self esteem is what I think you are looking at. Is she involved in some sort of activity (group activity that is) at some point during her week? If not, that would be a suggestion - find something she likes, and get her involved in it - there are a million things to choose from. She needs to feel needed and wanted by others. If she doesn't find it there, she will look for it in other unhealthy ways - as I see some of the other mom's mentioned bad relationships in their younger years - I too struggled with that - you have the desire to be loved and needed by others so you throw yourself at other people. (esp. guys in your teen years)
I hope you find something soon that will fit the bill for her - just explore what her interests are, for example if she likes to read - perhaps look into joining a program to help younger kids read...something like that might be good.
The meltdowns are her cry for help - so do what you can to help her now before she gets older, because repairing the damage when she's older will be harder. Let me know if I can help in any way! D.