Complicated relationship advice...HELP!?

I'm currently in a on/off relationship with someone and when we are 'on' I wish I was single and free sometimes, and not in the pressure of a relationship, but when i'm away from her I miss her so much. We talk and text ALL day, and I know she misses me just as much, we are like best friends and are so used to telling each other everything, that we just cant seem to go our separate ways completely, for good. But I couldn't bare to see her with anyone else, it would break my heart, but other times Im just not sure if we are right for each other...and she also feels the same. Yet when we are 'off' we miss each other so much and end up back together getting on better than ever.. I don't know if its the commitment we are scared of, or if we are just both too insecure to be alone, or wether we just both want to have our cake and eat it!...We are both giving each other so many mixed signals, yet even when we say lets just get on with our lives separately, we somehow cant leave each other alone! We've both had turbulent upbringings and have issues because of this I suppose, but I cant work out if that is what draws us together....or causes the insecurities that cause the arguments, that eventually pushes us away! When we get on we are like twins, but when we don't, its war...! We cross the love/hate divide regularly and it wears us out, but we still end up making up..like brother and sister almost...one minute we hate each other the next..we're cuddling! and visa versa...i'm so confused...i don't know wether to just be strong and fight for this relationship, or be strong and leave it! help please!!