There's a popular saying that goes: "Men will come and go, but your girlfriends are here to stay." Though your girls might at times let you down, true friendships that are nurtured properly can last forever. Part of living your best life is being a good girlfriend through thick and thin. It's not easy, but it's worth it. Need a little inspiration? Here are 18 must-read quotes and mantras that honor the importance of sisterhood.