As Major said this has been well discussed here. However, I certainly don't mind repeating myself as this is such an important subject to have peace on.
After meditating on this for many years,
my current understanding is as follows:
Believing we can lose our salvation is supported by lots of scripture. Believing we cannot lose our salvation is supported by lots scripture.
Good start hey.... (if you must know, Kevin is to blame for that opening).
Now putting the two together with some common sense. It takes sinking to the
depth of our hearts intent to properly make the decision to accept Jesus. We are saved when we are saved, not when we say the words, though saying the words 'I accept Jesus' show we are on the right track. Like Peter had a revelation that Jesus was the Son of God. An undeniable spiritual encounter took place, as Jesus said 'flesh and blood cannot reveal it to you but only my Father in heaven'. This depth of intent is not a depth that we can always sink to or live at. Example. When we got married we were completely in love and fully convinced of the decision. The honey-moon phase was awesome and just confirmed for us that our decision was correct. After the honeymoon it is possible that we could suddenly wonder why we married the person. We start ignoring them and doing our own thing in the marriage. Does this mean that we are not married or rather that we are being bad husbands? We are merely being bad husbands. Hence the once saved, always saved teaching is very strong.
However my gut tells me that those believing once saved always saved believe it so firmly because they can't imagine a life without God as they are so
close to God. It is hence a good sign for me to hear someone say they believe in once saved always saved. Kinda of like a married couple in love, cannot even hold the thought of adultery in their minds.
Now the flip-side. In comparing our relationship with God to an earthly marriage we have to understand that God the Son is a perfect spouse. He will never leave nor forsake us. But we can leave and forsake Him. Is it easy to do this? Absolutely NOT. As I explained when we got saved it took tapping into a
depth of inner desire to accept Jesus. It takes an equal depth of inner desire to rebel against Jesus to leave Him. Example: Jesus says that if our spouse cheats on us, we will be justified to get a divorce but we do better to forgive them and take them back (note that it is the extremer evil of adultery and not merely cursing, lying, stealing etc, though those may be indicators that adultery is en-route). So Jesus clearly desires to go the extra mile and then some for us. I see this as applying only if our spouse had a once-off or few falls into temptation. Not a long lasting affair. A long lasting affair would show a complete change of hearts desire for someone else. Would Jesus say to a spouse who was been cheated on for a solid year that they must forgive and take them back or rather that the cheating spouse has already left them? We know that scripture says that it is fine to get a divorce if the other party leaves us. So you see, to me the extremity of the inner rebellion becomes the issue. So as a final statement I say that if we
continue in an
extremity of evil we can walk away from (not lose) our salvation and must be nervous of an eternity away from God. Now what Paul is saying is that someone who is capable of that, what hope have they got of coming back? near zero. We cannot ever take it as completely zero as we know that all living still have hope to come to the Lord and if we sincerely repent He will forgive us 7x70 times a day. But would someone who has sunk to such a level of inner rebellion truly repent anytime soon? That is what I gather Paul is referring to.
So regarding your predicament specifically, you would need to ask yourself, did you 6 years ago have an undeniable spiritual encounter with Jesus? If yes, judge the level of your inner rebellion. But whether it was extreme or not is actually now
irrelevant as you are clearly back with your first love. God is smiling at you and has forgotten your past as far as the east is from the west!