I think a good start is to figure out why you feel jealous. It can offer valuable insights into your feelings.
The most common cause is insecurity. You can work to deal with that by realizing that she wants to be with you. There is no reason for her to leave you unless you two are not working out. Her relationship with anyone else does not affect the love she feels for you.
A big key to making these relationships work is communication. It is ok to talk about how you feel. You can say you feel jealous or insecure. But that does not mean she has to change anything. It may prompt her to explain why she values you. Or she may decide to spend more time doing an activity with you.
The whole key to polyamory is that you are with someone because you want to be with them. You are not tied to them. You are free to feel for others.
I would suggest reading other posts here. It may give you a better idea of how different people approach these issues.