Once saved always saved
I believe you cannot be unsaved: John 10:27-30 (Jesus is speaking) "My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one can snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father's hand. I and the Father are one."
So, in essence, NO ONE can take you from Jesus hand once you are saved, that includes YOU. We are not greater than the Father, therefore, we cannot pull ourselves from His grasp. That is a very conforting knowledge, because our salvation is assured.
"It is by grace you are saved, through faith, and not of works, lest any man should boast." It would take works if we had to retain our salvation in any way. We were dead in our sins before we accepted Christ as our Savior - and what can a dead man do but lay there and rot - it is only through the grace of God that we are awakened from the dead through faith in Christ Jesus - not of our own works or deeds.
So, what does that have to do with the parable of the seeds? I believe the seeds are different stages in a Christian's walk. There are times when we blossom fully under Christ's daily watering and nourishing. There are times we try to 'go it alone' under our own power, and become the seeds that fall to the hard ground. The condition of the human heart is very fickle, and it is when we look away from Christ that we fall. But He is faithful always to put us back on track, to pull us closer to Him, to continue to form us in His image.
No, we cannot ever be unsaved - we can be a prodigal child, but in the end, Christ will bring us back. "Train a child in the ways of the Lord and in their old age they will not depart from it" - it doesn't say that every day and every second we will walk the walk Christ would have us walk - on the contrary , we will fall, we will sin, but Christ is always there to cover our sins with His precious blood.