Well you don't say exactly what it is that you want help with. You say that you two have been going through some rough times, so take it you've been arguing. I don't know what kind of situation you're in, but something that every relationship needs is communication. Have you two talked about you're issues? If not then I would start talking. As for the parents, well, you've been dating their daughter for almost six months and they STILL haven't met you? Meeting the parents can be uneasy but its one of those things that have got to be done. That's probably one of the reasons why they don't like you because you haven't made the initiative to meet them. Someone dating their daughter for that long should have done so by now, because in a way it kinda sends a negative message about you if you don't. I also agree with LindaRuth, they don't like you because you make their daughter unhappy. Or it could be that they see that you have a negative influence on her.
I would talk things out with your girl, and if you two stay together, then I would meet her parents ASAP.