Our belief does not make Jesus faithfulness real.
Right.
Your belief makes Jesus' faithfulness
applicable to you.
Faith is how you access the faithful and never ending ministry of Christ (Romans 5:1).
That's why you have to continue to believe in it to continue to have the benefit of it.
That's what he told the Galatians.
Just the opposite. The faithfulness of Jesus is the basis of our faith and makes our holding fast the hope of our confidence to the end a reality
Believers do not lose the choice to hold fast to Christ in faith.
The saved Galatians exercised their freedom to depart from faith in Christ for justification and went back to the law for justification.
And Paul expressed concern that he had wasted all his effort on them.
Which shows us he did not believe 1) that they remained saved despite their rejection of Christ, or 2) that they would categorically and without exception come back to faith in Christ.
See, in osas, there would be no consideration whatsoever about having wasted time and effort on saved people who fall because they always, in the end, will be saved. Obviously, Paul did not believe that.