Why do I always think I am dying of something?

Hello. I am a twenty year old woman who for the last eight months has suffered with severe anxiety mainly regarding my health and worries.
Over the past month alone, I have been to my local GP surgery four times - two for my heart palpitations, and the other two for my left breast and fear of Breast Cancer. I have an echo-cardiogram on Thursday which will hopefully rule anything bad happening with my heart out, so I can finally stop worrying so much. Regarding my left breast feeling bigger, harder, and more pressurised than my right, I have to go back in two weeks time to see if it is hormone related, as no lumps were found, and my lymph nodes were fine.
Just these two health worries alone have nearly mentally driven me to the end emotionally, and are also upsetting my family a lot by their seeing me in such a constant state of fear.
I currently have a pain in my left neck lymph node area which I am convinced means a cancer has spread from my breast, and is moving up and around my body. My anxiety is so irrational, yet it is still destroying my life.
I would love to hear from anyone about how to cope, or rationalise my thoughts and fears? I am so tired of living life scared of something I am making real in my mind, and thankfully not in reality.
Any responses would be much appreciated.

It sucks and I feel the exact same as you. It seems so irrational because I can fear something different every day. One day it's melanoma, next heart problems, brain cancer, blood clots. It's stupid. I'm 21 as well, so basically the same age. We shouldn't have to worry about this stuff all day because it really makes no sense. It's funny because it's so easy for me to tell that to someone else but so hard for me to believe it myself. My health anxiety started around 7-8 months ago as well, and has made me a wreck since. I wouldn't wish it on anyone.

Thank you so much for the reply :)
Like you said, I wish there was more I could do to comfort you and others although believing it about myself is so hard.
I have wondered over the past few months about what 'normal' people who don't have health anxiety think when they have a headache or a sore throat - probably that it is nothing serious. Then there is me, thinking I have some form of disease that is hidden or suddenly appearing.
I have always been a worrier about a lot of things, but mainly my health. It has never been this bad though, where some days I just want to lay down on my bed as it is the only way I know that definitely calms me down.
I am really sorry that you too suffer from health anxiety, and I hope you find a way to manage it sooner rather than later, as we're both too young to be ruled with worry and fear.
I guess we have to find some positivity and live in the moment rather than worry about the 'what ifs'.
Good luck, and am happy to have heard from you.

No problem! :) I used to feel normal, and then I had my first real panic attack and thought something was really wrong. Since then, I've had this anxiety problem. It's ridiculous. If my leg was bothering me a year ago, I'd have shrugged it off and moved on. Today, I keep wondering if it is something serious and I'm ignoring it. Always a new symptom to worry about. I hope you figure it all out as well, we don't deserve this!