I can't advise but I can tell you what it was like for me. My son's relationship broke up when my grandson was two and the little chap went between his dad's and mum's houses three times a week with his tiny backpack on. It almost broke my heart and at times I felt overwhelmed. I looked after him endlessly during the days his dad was working and consequently we're very close.
He's 16 now and has continued to inhabit two worlds (his mum is hippy-ish, his dad more of a businessman) all this time, enjoying the variety. He doesn't know any other way. He's a lovely, wise, grounded teenager. Anyway, that may not console you now because the future is a long way off, but believe me I do know how you feel. 🥀
When his mum asked him the other day what he remembered about his childhood we thought he would mention his two lives, as he is very honest and open, but he said, 'Nana's metal tape measure that used to zip back in really quickly like a snake.' 😂