I'm jealous that my best friend is dating someone
Why are you jealous of your best friend? Are you secretly in love with them?
No, I'm jealous but I know that it is futile to let myself be affected by the matter
I can understand how that can make you jealous, but you still have to be supportive of them. You are friends and you are there for each other good and bad. It is in bad spirits and it may dampen your relationship if you just continually resent whatever nice happens to them.
What is so hard about being nice? I'm going to be petty and annoying, should I?
You should try not to be so bitter about their new found romance. Your friend is happy so you should be happy with and for them. One day when you find love too, you're friend will reciprocate that same empathy.