What does being crucified with Jesus mean?

I'm confused. I listened to a sermon which was about being crucified with Jesus.
The pastor said that we shall no longer focus on sin because sin will become less and less. This already made me a bit sceptical. And then he talked about Paul saying that he was crucified with Christ and that he is dead. And then he said that sin is defeated and a human who has died with Jesus knows that sin is defeated and that you don't have to try to control sin anymore.
He said that people which have not understood that they have died with Christ will always look at what they have still to improve and which sins they still have and try to control their sin.
Is he right? What upsets me is that he said that the bible says this and he made it look like if you don't believe what he says then you don't believe the bible!
But if this was really true then all christians which finally understand this would be free from sin and not sin anymore because a dead man cannot sin and somebody who has died to sin also cannot sin anymore but is this what reality shows us?
Is there any christian who doesn't sin anymore? Did Paul not sin anymore?
I cannot really believe this. To me this stuff sounds very strange and unrealistic kinda like you can evolve into something better, into something totally christlike where you will be totally free from sin if you only understand and believe what the bible says....
This seems very strange to me. But on the other hand I also ask myself what does the bible mean with being crucified with Christ? If I have died to sin then why can and why do I still sin?
I also don't know how to become really free from some sins and I find myself struggling with them. What if this is also wrong and there is indeed freedom from sin and God doesn't want this stuff where you feel bad because you sinned and then you say to God that you still have to change this and that and this and that and always feel guilty.