Getting 5 week old to sleep flat

I feel like I’ve tried everything. She is swaddled, fed, doesn’t have burps left, clean diaper, etc. I cannot for the life of me get this girl to transition from sleeping on me to sleeping in her bassinet. As soon as her little head lays down she is squirming and shortly after is screaming. My mattress is too soft so lately I have resorted to holding her all night but I am so tired. I had to sleep in my sons room on a mattress and cosleep in order to get him to sleep and I really don’t want to do that again. Any advice?! Please! 🤞🏼🤞🏼