Since Eternal Salvation is eternal, how could you lose it?
Salvation is indeed eternal, as stated in Heb. 5:9, but please note who receives it:
And having been perfected, He became the author of eternal salvation to all who obey Him.
Obviously, eternal salvation is only for those who obey (continuous-tense) Jesus!
This does not mean, however, that perfect obedience or sinless perfection is demanded for salvation (1 Jn. 1:8; Jam. 3:2). But again, we cannot compare the lesser sins such as worry and unthankfulness to sins such as sexual immorality, drunkenness, greed, etc. that will exclude anyone from the kingdom.
Therefore, based on Heb. 5:9, if one doesn’t obey God, he should not think he will enter the kingdom of God one day. The Lord Jesus taught the same in Jn. 5:29:
And come out—those who have done good will rise to live, and those who have done evil will rise to be condemned.
See also Rom. 2:7 and Gal. 6:8-10.
One final point, the only time we read of our eternal salvation is in Heb. 5:9, yet when the Once Saved Always Saved people mention our eternal salvation they almost never cite the entire verse which shows who the recipients are!