HELP 2 MONTH OLD DAUGHTER CONSTIPATED =(?

Hi my 2 month old daughter has been constipated for a month now and her pediatrician has told me to give her a ounce of Prune juice and it will help her push easier. I tried that & it worked but besides that shes still constipated , i really need help most parents know about this & has experienced it, but my pediatrician told me to try giving her a ounce of water mixed with a T spoon of brown sugar but thats if shes still constipated so i did and it didnt work at all she neva made a bowel movement. My daughter constantly cries with tears in her eyes from bad gas and i believe so that her muscles arent strong enough to push on her own to make a bowel movement. The pediatrician has told me its okay to use a thermometer with a little bit of baby vaseline to help her push and i have been doing that and thats the only way it works for her to make a bowel movement cause she starts to push. =( please help meee i dont wanna have to keep using this thermometer on her & end up hurting her everytime she has to make a bowel movement. Everytime she has to go , the only way of helping it come out is the thermometer. someone pleaseee help meee