Hebrews 8:8 For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah:
9 Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord.
10 For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:
11 And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest.
12 For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more.
Here is where Paul stops quoting prophecy and adds his comment. The "new covenant" he is speaking of is the one in effect as he speaks, he is simply interpreting prophecy, not suddenly adding that this new covenant he was in then was waxing old. He is referring to the passing of the ancient Jewish covenant which was crippled and about to be brought to judgement, to vanish away in AD70.
13 In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.
The proof of this is the very next verse in Hebrews 9, Paul continues in the same vein, describing the old covenant, what it consisted of.
Hebrews 9: 1 Then verily the first covenant had also ordinances of divine service, and a worldly sanctuary.
He was never referring to the passing of the only new covenant then in effect as if it was somehow ready to vanish. We are in that very covenant he was speaking of.