Lately i get nervous/angry/out of me easily

Lately I get irritated very easily. I consider myself a calm person, but I have had some discussions with my boyfriend and lately when we discuss I get very angry and out of me, I also say things I don't want to, because I'm so tired of arguing.
Sometimes I reached a point where it seems that my head will explode of so much irritation and nervousness.
We already have solved the things, and we are better now.
But today arrived my sister (who is older)-- i'm 18. she is really unbearable. She has no respect for me, and always tries to annoy me. We always fight when we lived together. and even now, so long without seeing each other, she still pisses me off with what makes me and tells me.
In an interval of 10 min she came into my room like 5 times just because she wanted to force me to do what she wants.
I just want to be alone in my room, but she always enters without knocking, do what she wants, even after I have asked her to knock the door and tell her I want to be alone. She just ignored me and keeps doing the same and trying to ordering me.
Now we had a fight, and I'm with my hands trembling -- wich is not usual-- and really nervous.
I'm afraid to start being a very nervous person and that become bad for me, and for my health and sanity..
I want to be a calm, passive and patient person. I really want.