It seems like 15-18 months can be a hard time for nursing Mamas! This age group is pretty needy anyway, and can easily make Mama feel 'touched out'. I have heard that this phase does pass! 
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It may help to remember that you can set limits - maybe night-wean, or limit nursing during the day. It doesn't have to be all or nothing. Winter is a hard season to wean. I have a friend with a 20 month old who is also "so very done". She's setting limits but will wait until summer to wean fully - after flu and RSV season is over.
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If you aren't certain about weaning you can always take it slow. Cut out the most annoying session, or the one your DS is least attached to and see how it goes for a week.
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My little guy is only 9 months, but I gotta say I am looking forward to some better weather! All this snow makes it hard to get outside and that does nothing for my mood. (Off topic I know, but I saw that you are in ON so I thought you'd understand!)
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