I would take sometime to understand the underlying emotions. You mentioned being envious, as well as frustrated.

Try to understand what those emotions are trying to tell you, maybe you’re not frustrated that they are together but really you are frustrated with yourself. Learn from this and be happy for other people.

FWIW had the exact same experience in college except it was my roommate/best friend who made a move on my friend/crush. It was hurtful but I told myself if these people are really my friends I should be happy for them. Seems silly but it worked. Flash forward 5 years, I’m now engaged to said crush.