One big slip and I feel all my years of prayer, fasting and good deeds are wiped out and worthless. Is one sin really enough to erase everything?

No, one sin does not erase a lifetime of worship. The feeling is real and it is heavy, but it is not how your Lord keeps His accounts. Let me show you what He has actually said about it.

Allah has told us where the real strength lies, that the good is stronger than the stain that falls on it:

“Indeed, good deeds erase bad deeds.”

Qur’an, Hud 11:114 · meaning

Read it slowly. It is the good that erases the bad, not the other way around. Your years of prayer and fasting are not fragile things waiting to be shattered by a single stumble. They are a standing treasure, and the believer’s way after a fall is not to mourn it but to answer it with fresh good.

The Prophet, peace be upon him, gave the simplest medicine, a habit any heart can carry:

“Follow a bad deed with a good deed and it will wipe it out.”

The Prophet (peace be upon him), Sunan at-Tirmidhi (1987) · meaning

See it as it is. A slip is one dark drop, and a sincere good deed is clear water poured after it. Those who study how habits are built say the same thing in their own way, that what shapes a person is not the rare stumble but what he does in the very next moment. Your faith hands you that next moment. Do not spend it in despair, spend it in a good deed.

So set the heavy feeling down. Nothing worthy has been erased. Your prayers still stand, your fasts still count, and the way back is as near as your next small act of good.

One small step: The moment a slip weighs on you, do not sit in it. Do one small good at once, give a coin, say “SubhanAllah” ten times, or offer two rak’ahs, and let that good be the water that clears the drop.