Beo, it is good to hear from you.
When you find yourself focusing on whether or not you are making her happy, and the idea that you should be the sole source of her happiness, just try to remember that we are all each responsible for our own happiness. It isn't up to anyone but ourselves to make us happy. Jen's happiness doesn't solely rest on you, and your happiness isn't contingent upon her. So, having more than one love in her life simply means that that enables Jen to best express who she is, which brings her happiness. It's not the people that "make" her happy - it is the freedom to satisfy a need to be herself.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein