4 months old baby won't stay asleep at night..? HELP!?

I have a 4 months old baby and she doesn't know how to stay asleep at night and when she does she wakes up every 2 hours literally ! She is co sleeping with me and I am breast feeding also. Please help me I don't know what to do any more .... I've become so tired ! This been going on for a week now.
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