I struggle with church in so many ways but this one could be the biggest one that I see that hinders it the most.
The old covenant is over and the new covenant takes it’s place as is now the Covenant that is God’s promise to us.
Let is look at this closer, church I believe has seen or heard of the new covenant but has relied on the old covenant to live. We try to instil the principles of the old covenant into the new covenant to continue the promise of the Lord. But this is not the same.
There are many examples that we can choose from, just look at our practices of faith, and our works, do they mirror the ones of the jewish nation as to going to the temple and having sacrifices, or having ordinances, or regulations that we have to follow.
The old testament was an example of a people trying to be a people of God, thru living the laws of God, which were only there to prove to them that they would be unable to follow because of their fallen state. We were not ever going to be perfect enough to follow the laws of God. Our nature was and is not ever going to allow this.
So something or a new nature had to come in to replace the old. God’s people, the Israelites, and even us today have mingled both covenant’s together thinking that this is the way to precede.
We need to know that the first one is dead, the new one is alive! Because Christ is alive, the law is dead because it does not lead to Life.This is so important to understand in many ways, which we will only touch on one now.
Let’s look at a verse that will be glowing to us in this understanding of the new covenant.
Matthew 17:4 (Recovery Version)And Peter answered and said to Jesus,Lord, it is good for us to be here;if you are willing,I will make three tents here,one for you and one for Moses and one for Elijah. 5 While he was still speaking,behold,a bright “cloud overshadowed them, and behold,a voice out of the cloud, saying, This is My Son, the Beloved,in whom I have found My delight. Hear Him! 6 And when the disciples heard this they fell on their face and were greatly frightened. 7 And Jesus came to them and touched them and said Arise, and do not be afraid. 8 And when they lifted up their eyes, they saw no one except Jesus Himself alone.(Recovery Version)
To understand this passage we need to understand Peter and the disciples at this moment of time. To understand peter and the disciples in this, is first to understand the symbolism of Moses,Elijah and Jesus.
Moses,was symbolism for the Law, Elijah, was the symbolism of the Prophets, and Jesus, was symbolic of the new covenant. Peter,disciples were mixing all three of the covenants together as tho they were to work together as the new. But no, God’s intervention into Peter’s building of three tents to three symbolic periods was meshing of the old and the new together. The old was over, there is only one now. Even Peter and the Disciples had to understand this.
Look at verse three, then look at verse eight, now you see three and now you don’t. you only see one, Jesus.
The New is now and has been for awhile since Jesus Resurrection.
I pray that we as “Overcomer’s”, that we can see one and only one thru our eyes,Jesus.
May we live out our new understanding of the “New Covenant”!
The Eternal Purpose 
You are right that we often mix the old and new covenants and tend to live with one foot in each. Hebrews 10:9 is clear: “He takes away the first that He may establish the second.” He did not add the second to the first.
Do we then forget the law? On one hand, yes; it is the first covenant and has been taken away. On the other hand, we are not lawless, for Romans 8:4 says “the righteous requirement of the law might be fulfilled in us, who do not walk according to the flesh but according to the spirit.”