How can I be a good boyfriend?

I dont know whay but its like im just not good at being a boyfreind. i went out with this girl and we had a great connection and for the real reason i know she wouldnt tell me we broke up 2 weeks into it all. i had a diff gf and same thing, nvr got a real reason why, i just dont know what to do or say or how to act or what. its been buggin me lately bcuz i hava thing with this girl tht as far as we can tell now we like eachther. shes rlly sick and i jst want her to feel better so i can see her. every time she mentions bout how shitty she is feeling idk wut to say back rlly. can u giv me advice on how to be a bf not wut ever i hav been to my past gfs and what i can say to her like wen she talks about how sick she is or wen she talks bout her emotions in general.