It sounds quite simply that you aren't getting your needs met. She has a lover, it sounds like you don't, she had a threesome, you didn't, she is barreling forward into poly, you aren't.
I would be patient, work on finding someone you can love, without seeing it as a rush or competition. New relationships shouldn't be forced I think . Your turn will come eventually, but in the mean time you could learn from her because of her experiences and it could make you better prepared for when you have a relationship all your own.
I would be careful not to make her feel bad about your jealousy, keep supporting her, loving her and being her rock in her situation.
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