Can someone explain why did she forgot about her childhood? Or did she just decide to close everything from Middle school to when she was little?
She didn't forget, she actively remembers in the preceding panels, but says that because she doesn't want to give the real reason as it would hurt her mom, and Saki has shown time and again that she will just internalize pain and push past with a smile, never voicing her own feelings, so that others are not hurt which makes her the flipside to Kanon's outspokenness.
It's pretty clear her dad was already gone very early, assuming that was her around age 4-5 and considering her grandmother's funeral was the first one she recalls, and her mom having to both work and be the caregiver for the grandma meant that Saki was somewhat neglected and lonely as a child, hence the juxtaposition of seeing her mom's back as she walks away with memories of being alone at the park watching other kids play with their moms.
My guess is that her mom being so busy Saki never asked for any friends to come over, and, honestly, we really haven't seen any sign of friends in Saki's past. It really looks like that starting piano lessons at 7 which was likely for her to have something to do rather than just stay home alone, which we know she does rather than going to school clubs, clearly figuring out her attractions by late elementary, further marking her as different and unable to participate in a lot of the stereotypical talk with other girls, and whatever seems to have happened in middle school, almost guaranteed to be negative since we know Saki hasn't dated anyone, and it seems that for very different reasons Saki was as socially isolated as Kanon, just better at papering it over with a bright and extroverted personality at school and having some classmates she got along with.
So, it's very likely Saki hasn't really had a close friend, or any strong friendships, till Kanon. Certainly not someone she would spend time with after school.