Sometimes Sirius still gets confused on if his mother loved him or not. Yes she was abusive and emotionally manipulative, but sometimes . Just sometimes she'd give him a warm hug and apologize for her harshness. Sometimes she would tell him just how proud he made her. Sometimes they would dance together at the pure blood parties.
Sirius knew she wasn't the perfect mother, she wasn't even a good one. But that night he ran away to the Potters with tears in his eyes he knew he couldn't look back because the second he saw the piano in the giant living room that his mother used to teach him how to play, or the balcony that still had the telescope she used to show him exactly where his constellation and star were. He would've stayed. He would've stayed and endured the pain because when she good she was so good.