Hello I Need Help

So I smoke months ago & ever since that night I been thinking I’m dying constantly it ended up stopping when I was around my fiancé a lot then once I left his house go come back home it started back up,, I keep thinking I have a brain tumor or I have a messed up heart,, I think negative stuff & I get those in my head & start freaking out.. I just wanna be back to normal 😞 i don’t like these thoughts