Faith is the assurance of things hoped for, the evidence of things not seen.
We know that we have assurance because God said so and He is trustworthy to keep His promises.
Feelings are nice, but are NOT the barometer of our faith.
And we have this:
1 John 5:11-14 NASB
And the testimony is this, that God has given us eternal life, and this life is in His Son. He who has the Son has the life; he who does not have the Son of God does not have the life. These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life. This is the confidence which we have before Him, that, if we ask anything according to His will, He hears us.