You sin and sin but He forgives

Indeed, those who have believed then disbelieved, then believed, then disbelieved, and then increased in disbelief - never will Allah forgive them, nor will He guide them to a way.

[Qur'aan 4:137]

The Verse is evidence that if they did not increase in kufr, but returned to Imaan and gave up the kufr they were upon, Allaah would forgive them even if they had had repeatedly apostatized.

When such is the ruling regarding the one who commits kufr [he apostates and then  returns and then apostates and then returns and Allaah still forgives him despite his repetitive behavior] then when it comes to disobedience which is other than disbelief, it is only more likely that a slave even if he has repeated the disobedience and then repented that Allaah will repeatedly forgive him.

Reference(s):

Tafsir al-Sa'di

A person commits apostasy. Then, he believes. He again commits apostasy. Then, he returns to Islaam. He commits apostasy again but returns to Imaan for good. 

Allaah forgives him and accepts him.

This is the case with a sin as bad as apostasy. There is not a sin as bad as apostasy. 

If a person commits a sin, which is less severe than apostasy, then it is only more likely that Allaah will forgive him regardless of how many times he has repeated the sin as long as he repents with sincerity each time and with full resolve that he will not return to the sin.

If he falters, repents, and tries again, Allaah will forgive him in shaa Allaah. Allaah will continue to forgive him as long as he continues to repent sincerely.