Why do i have to jump start my dodge durango every morning and i have a new battery in it?

i have to jump start my 99 dodge durango every morning. we just bought a new battery for it. after we bought the new battery it was starting fine for a couple weeks. then back to its old tricks. it just wont start. i dont think anything in the car is left on every night to drain the battery. it takes awhile to jump start it. it has been pretty cold but i dont think that could be the issue since the temp is about 20 degrees. can anyone please help me. it makes me and my husband so frusturated. thanks