What quality do আপনি think Lily অথবা a partner could teach Severus?

I don't think আপনি can "teach" someone in order to change him/her. Especially when adults. Unless the person wishes to change and/or see an interest in doing so. The only change that can last is the one we initiate ourselves.

However, the example প্রদত্ত is of a great value. In this regard, patience and compassion indeed, would be useful for him. I mean, he would see it would be interesting to possess these qualities in his relationships with others.

Otherwise, maybe he could see how good it may feel to be forgiving towards himself first. A potential partner would have to প্রদর্শনী him forgiveness for himself. And to smile. Yes, important, smiling...

I believe she (or he, whatever) could teach him compassion, অথবা at the very least to rein in his temper.