My girlfriends sister came to visit and asked me if I planned on getting married .
I said yes in about a year .

Then later that night my girl friend told me that it bothered her that I told her sister that. I didn't have to lie to impress her sister and that I didn't take the relationship seriously .

I told her that I wasn't lying and that we have been together for 2 years now that by next year we should be married.
She didn't say anything.

After a week went by I thought everything was good .Then she texted me that it was really bothering her that I had said that.

Soi tried to explain to her again why I had said that then she told me that it was over and that I was only using her and she stopped texting me won't answer the phone.

Should I continue trying or should I let her go ?

I would suggest you work on communication.
I am unsure of what the problem is you refer to.

I think it is that your girlfriend does not believe you are serious about the relationship , in which case after two years together you both should know where you stand.

That is a definite sign of lack of communication.

You might try to better understand her but if she refuses to talk there is little you can do but move on.