part time mum ???

when a couple breaks up after they have children what is the best thing to do for the child, lots of things say equal time with both parents is best for the child, but how comes i dont now anyone apart from me that lets their child/children have equal time with themselves and their exs, does this make me a shit mum, because sometimes it makes me feel awful sometimes it makes me feel like a failure, why do i let my son go every other week why???? i cant even remeber what it was before this arrangement, how do i change it ?? can i change it, i dont think i can and sometimes it makes me feel sick, i should have him more time than i do, i should but why dont i ???