The following is a comment Shaykh Yasir Qadhi made on the Muslimmatters.org website which I thought was quite useful.
We need to define a kabeerah (major sin) versus a sagheerah (minor sin). A major sin, according to the strongest opinion, is a sin that entails Allah’s curse, or threat of punishment of Hell. So actual fornication is a major sin, but the steps leading to it (e.g., talking to a woman, looking at her, etc.) does not constitute a major sin. No doubt some ‘minor’ sins are more major than others, but they are still in the realm of minor sins. Hence, even if a sin is ‘disgusting’ it does not necessarily follow that it is a kabeerah.
Not to trivialize minor sins here, but we need to remain clear about these matters.
(Shameless plug: this matter will be discussed in much greater detail in a future AlMaghrib class insha Allah!!!)
Now, a minor sin is transformed into a major one when a person habitually does it without any remorse or regret. What makes it a major sin is the lack of guilt and lack of repentance – if a person continues to repent and then falls into a minor sin, it remains a minor sin. Hence, if a person *tries* to give up a sin and continues to slip into it, then that does NOT constitute a major sin. Only if a person loses all modesty and habitually commits such sins without any guilt or attempt to change will such minor sins be considered a major sin.
Hope that clarifies this issue insha Allah…