Welcome to the boards!
Jealousy tends to arise from a fear of loss. What, exactly, is it that you fear losing? The "exactly" in the question is the most important part, I think. You have to pinpoint the precise thing you fear losing to address it. A generic "I'm afraid I'll lose them" isn't exact enough.
When I had jealousy issues in years past, for instance, I wasn't afraid of losing the relationship. I was afraid that my partner at the time wouldn't spend as much time with me as I needed for a relationship. My fear was not having enough bonding time, and that might lead to losing the relationship eventually. I've heard others describe a fear of losing unique experiences, that a new person might get included in something that was formerly "our" activity (A and B do something that is "ours"--they don't do with anybody else--and the fear is that if A and C get involved, then A and B will no longer have that special activity to themselves).
What really helps things along is to figure out, exactly, what you need from the relationship, then negotiate boundaries that provide for those needs. Once you see/hear/feel exactly what it is you require, it's much, much easier to not worry about your partner's activities in the remaining areas. In other words, if you require some sort of close bonding three evenings a week to be satisfied, then knowing you'll still get that makes it easy to not worry about the other four evenings.
And remember that the process is ongoing. What's needed now isn't necessarily what will be needed in six months or six years from now. I don't require nearly as much time from partners as I did years ago, for instance.