No point waiting

Im 18 and i live with cystic fibrosis and diabetes. Im expected to live until 21 or 23 years of age. My brother is 19 with same conditions and cannot walk one room down the hall without being hooked up to oxygen systems and still getting exhausted. I had a special lady in my life we had been together since high school. We were 14. she was always there for me and understood things i went through and supported me. Today i found out she has been cheating on me for 2 months, including on my 18th birthday. She was the one thing i looked forward to in the morning but now i think she was only with me for a place to stay since her mum passed away. I will die in 5 years tops and life will become hell well before then and my one good thing has left me. Im scared of that time coming. i want to end it soon. I cant sleep and i havent eaten in 24 hours. I have felt this way on random days for over a year. Bu now it has gotten too heavy. I considered suicide few times per month but now the time is right i can feel it. There is mothing left for me here i am only a waste of space  I may as well speed up the process. I have nobody left to talk to but myself.