Nowhere does it say the seal of the Holy Spirit cannot be broken (read Heb 6:4-6). If you lose your faith, will you still be "sealed"? If you commit "deadly" sins (1John 5), will you still be sealed?
That word "seal" can also be translated as "mark" - you've assumed it is automatically permanent - it isn't.
There is some truth in what you are saying. Through the Bible we have the salvation promises of God as our "title deed". It was the same when I bought my new home in Christchurch last December. I paid the price and had to wait for four weeks until the possession date. I had the title deed made out in my name, so I knew the house was mine, but I to wait until I got full possession of it.
We have broken the Ten Commandments all our lives, whether we admit to it or not. If we have lied, taken something that does not belong to us irrespective of value, used the name of God as a swear word, and looked at a person with lust, as we all have, including me, then in God's sight we all are lying, thieving, blaspheming, adulterers at heart, and when we die, which we all will, then we will stand before God in the judgment.
We will all be found guilty as charged on that day, because we all are. The person who has rejected Christ will be sentenced to God's eternal prison with no parole. Those who put their trust in Christ will have Christ tell them, I took the wrath of God and the penalty for your sin when I died on the cross for you, so you can go free and enter into the joys of your Lord in heaven."
This is like being in an airplane and given a parachute. One person can believe in the parachute but not put it on, another will believe that the parachute will save him if he puts it on. If the plane gets into trouble and he has to jump, wearing the parachute will save him crashing to the ground on his face as terminal velocity.
Because death is inevitable for all of us, it is as if we are in an airplane that is going to crash at some point, we need to "put on Christ" through faith and trust, and demonstrate through our repentance from the works of the flesh that we are wearing the parachute of Christ. Then when we die and come before God in the judgment, we will be safe because we did what we needed to do to sincerely trust in Christ for our salvation.
I am aware of what the RCC teaches, and I have deliberately steered clear of the faith/works debate, because our difference of opinion about it won't make any difference in the judgment. We won't have our church to advocate for us there, because all church structures and organisations will be swept away along with the earth and the heavens. This is because the Scripture says that earth, along with everything on it, and the heavens will flee away, and all that will be there is the great white throne of judgment where we all have to stand before and face God.