How Could a Past Sin Be Forgiven So Easily? (Honest Question)

Assalamu alaikum, everyone. I came across a hadith where a Muslim woman who fell into zina earlier in her life was forgiven just because she gave water to a thirsty dog. That really shows how vast Allah’s mercy is, subhanAllah. But here’s what’s on my mind: staying away from zina and other urges is one of the toughest tests from Allah, especially for us young brothers. So how come she got forgiven so easily? And could the same mercy apply to us too, inshallah? I’m a brother trying hard to lower my gaze and stay clean, and honestly it’s a major struggle right now. At the end of this tough road, it would feel kind of discouraging if the simplest acts of kindness can wipe away such major sins while I’m here stressing myself every day. Would love to hear your thoughts, barakAllahu feekum.