Is it normal being selfless and submissive (family and love issues)

Hi. I am quite an introvert and i've been wanting to share my problems with someone but i just cant because no one understands me. This is quite long but i hope you can keep up.
I am 18 and currently in a below standard of living. I have four siblings and my mother whom is getting older and sick. I am the youngest among us and i am only a step sister. My brothers and sisters arent married yet. I had to stop my second year in college so that my brother could study for his nursing license exam. And things might sound strange but i have to tell you real things that happen in real life. I am expected by my family to marry a 53 year old man from Australia and work there, and send money to my family so that i could uplift their state of living. For the past years ive been beaten up and even stabbed by my other brother. But these people are my priority and i need to help them.
I currently have a boyfriend whom is from a family that strongly follows the arranged marriage planning. They have this caste system and they are a decent family. I can say were pretty serious and we've met about a year ago. Some may think its all just a fling because we've only met through internet. He is very far from me. But we sacrifice every night just to see each other online. We love each other very much, and though he values his family and tradition very much, he still thinks of visiting me in the future. He is working hard for it. and we know each other better than anyone else. we fight, we share our pain.. we cry together..we’re like best friends.

Now i am very sick and i have serious health problems. But i have to push through with all these. I cry myself to sleep and think of just killing myself because i have no other options. It may be easy to say "oh just run away from your family and go live your life". But in our country that is not easy. Family comes first. and i love my mother very much and i want to take her pain away. I , as the youngest, have to submit my whole self to an old man whom i dont love. and he'll take me, and give my family a life. I am a brave person and i know i can do this. I will, for my family. but.. my emotions..its very hard when youre hurting but cant show it. how long would i have to hold it inside. i have dreams.. i have goals in life. I dont know what to do. Oh yes i would, im strong enough to marry and live with an old person, just as long as im giving my family a good life. but how about me.. how about who i am and what i want.. havent i given more already. Does my future matter at all.