I keep committing the same sin, then repenting, then falling into it again. Doesn’t my tawbah become a joke to Allah after the hundredth time?

No, it does not. You are imagining a ledger, and a Lord who by now has grown tired of you. But the Prophet, peace be upon him, was permitted to tell us of a servant who fell, repented, fell again, and repented again, more than once, and each time his Lord received him.

“A servant committed a sin and said: O Allah, forgive me my sin. Allah said: My servant has committed a sin and knew that he has a Lord who forgives sin and takes to account for sin. (This happened three times, and then Allah said:) Do what you wish, for I have forgiven you.”

Hadith Qudsi, Sahih al-Bukhari and Sahih Muslim (Bukhari 7507 / Muslim 2758b) · meaning

Notice what is actually being taught. Allah is not remarking on the sin. He is remarking on the heart that came back. Our scholars explain those closing words with care, and their explanation sits outside the narration itself: “do what you wish” is not a licence to sin, it means that so long as the servant keeps turning back, this door of forgiveness keeps opening for him. The one who keeps returning is the one who still knows he has a Lord, and it is that knowing which Allah is honouring here. Your hundredth tawbah is not a joke. It is evidence that faith in you is still alight.

There is a trap in this, and people who study human habits have even given it a name: one small slip makes a person think everything is ruined now, and so he abandons the whole effort. But the journey was never ended by the slip. It was ended by the despair that came after. A slip is a stumble. Despair is sitting down in the road and refusing to walk.

And against that sitting down, Allah Himself calls out:

“Say: O My servants who have transgressed against themselves, do not despair of the mercy of Allah. Indeed, Allah forgives all sins.”

Qur’an, Az-Zumar 39:53 · meaning

So the falling is not who you are. The rising is. Every time you turn back, you are doing the very thing your Lord invited you to do. Do not stop returning, and by His grace the habit itself will loosen its grip in time.

One small step: The next time you slip, before any other thought can settle, say once, “O my Lord, I have returned”, and do not let a single stumble argue you into stopping the walk.