Can you lose your place in heaven?

Something I've been struggling with is where this personal relationship stuff with God comes in. Maybe this is a stupid question but..I thought you were saved when you believe that Jesus died for your sins and that repentance is the only way toward heaven and redemption. And then you actually repent. That's what makes Christians different from demons; even demons know that Jesus died on the cross, but they don't repent for what they've done. So..I understand the striving to become a better person, because that's becoming more like God since we were made in his image. People say, write, and I read that without a personal relationship with God, there is no spot in heaven for you. But...I don't understand. I DO believe that Jesus died for my sins and that I have to repent. So where does this relationship stuff factor in? Is it like...another step on the way toward going to heaven? Or is it not enough to simply believe and repent, you have to do all these other things too? But isn't that doing works to get into heaven? Can someone explain this to me. Or tell me where I can read and find out more. TY.