I don't know what to do (relationship advice needed asap)?

I have been with this guy for a while now.. And his nice and stuff but there is this other girl who keeps coming and crying on his shoulder, and his always there for her, They are really good "friends" etc. a couple of weeks ago my best friend and i were at maccas, the girl who always hangs around my bf messaged my best friend on facebook, "I'm planning to lose my virginity to him". This made me uneasy about him hanging around her, i told him about it. he said it wouldn't happen and that he loves me not her. Recently he had ask the other girl to go to the park, and they did that. Next thing i see is posts of him being tagged in photos with her at the park... They look so happy and cute together... I am not sure what i am supposed to do anymore :/ should i just leave?