Pregnant again and trying to ween my 15 month old off breast milk

I’m going out of my mind, I’m exhausted as I’m in my first trimester of my second pregnancy and I have a 15 month old who still wakes for night feeds every night anywhere from 2 to some times 5 times a night. We are trying to let her “cry it out” but I can’t go any longer than 15 to 20 min with out feeling like I’m torturing my child. The doc says I should try and ween her off by 20 weeks to allow for enough nutrients for my developing baby, I’m 6 weeks pregnant now and I have no idea how I’m going to ween her off and get my sleep back. Any advice would be amazing!!