This is called taking one meaning to imply that something else written in another place , does not count .
No, that's not true.
But one passage will NEVER negate another, nor change it's meaning into something that's not written there. So when we find what appear to be contradictions, such as this, we need to consider more deeply how it is that both are true, and not in conflict, without changing one.
So in this case, we are looking at passages such as, "you have become a partaker of Christ if you hold your confidence stedfast to the end", and, "now we are children of God . . . when we see Him we will be like Him".
You are suggesting that we take the seeming conditional view of the Hebrews verse, "if you hold your confidence" to mean that we can cease to be a partaker of Christ if we fail to maintain our confidence, and that we should bring this understanding to 1 John, and where we read "we will be life Him, for we will see Him as He is", we would need to change this read,
1 John 3:2 KJV
2) Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know
(Not "know", maybe, "desire") that, when he shall appear, we shall be like him
(IF we continue in faith); for we shall see him as he is.
We would have to strip the promise of it's certainty.
2 Peter 1:2-4 KJV
2) Grace and peace be multiplied unto you through the knowledge of God, and of Jesus our Lord,
3) According as his divine power hath given unto us all things that pertain unto life and godliness, through the knowledge of him that hath called us to glory and virtue:
4) Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.
His promises to us are how we partake of His nature. He has given us all we need, but will we believe Him? I suggest to be careful of doctrines and ideas which lead us to change what we read into something it doesn't say.
So what about the Hebrews verse? It tells us that those who were made partakers continue. If you don't continue, you didn't become a partaker. That's the simplicity of the verse. You have become a partaker if you continue in your confidence. If you don't, you have not.
Much love!