I'm super jealous

So my boyfriend has this girl who supposedly his cousin not really she's like 16 or 17 and keep in mind he's about to be 20 and she is just all over him and I'm a jealous girlfriend because this guy is the first guy to treat me right and I don't want to lose him but how can I overcome this sense of jealousy cause it can turn into something very exaggerated, but let me continue this she was at his house today and she was laughing out loud and being touchy touchy and I wanted to tell her to simply back off but I couldn't despite not being rude 😩