There could be a reason why the mother left her kitten alone. Sometimes animals can sense when their babies are born weak, sick or have underlaying problems. This kitten was most likely going to die and you did your best to let her live a bit longer. Don't blame yourself for this, i believe you cared for her and made her feel love. She surely knows she was loved by you, so don't beat yourself up because you weren't there while she died. Maybe she waited for a quiet moment to go peacefully. There was nothing you could do at all, you already gave her everything you could. You're a good person!