How do I fight off jealousy?

That guy was not the one, even though it may have seemed like it at the time. Probably should continue to be amicable with them both. Everything happens for a reason.

Perhaps try not to be around them too much until you get used to the idea. A little distance will help you deal with it without feeling upset. It is fairly understandable to feel a bit jealous if you really liked the guy, but it is nice you are happy for them. I would try to just be normal around them and give yourself some space to get used to it.

I agree with @Mz_Lizzy Step away from the situation and give it some time.

This is just part of life. I know it sucks, I have been there. It will turn into your best friend having sex with your ex a week after you break-up. It gets harder and it sucks. Pretty much everyone has been there.

All you can really do is focus on what makes you happy instead of dwelling on shit that is out of your control.

No matter what you do it won’t go away.
Trying to make in unseen won’t work because eventually it will be seen.

You could just tell to your friends in different circumstances.

Or is it different ? You feel jelous on the situation and not on them ?

Maybe the real purpose of this guy liking your friend is so that you can learn to control your emotions. If you don’t, you will lose a friend and gain the reputation of being a jealous witch. The value of learning to manage your feelings will outlast their relationship.

Take a step back and look at the bigger picture. Don’t just look at the trees, look at the forest. This is one of life’s lessons. If you can over come these little hurdles of today, it makes life so much easier as you go on. Try practising empathy. To be understanding and loving to yourself, recognise where you are now, see what you have to over come and take steps to go in a positive direction. In practice this is hard, but over time it gets easier. Everyone has their own path and yours is the one you should focus on. Others have their own issues to deal with. Think in a year, two years or five years time… will this situation be a big deal? If no, then let it go, if yes that means you have issues that you need to address.

Just be happy with who you are and what you have. Also continue to be happy for your friend.

Find someone else.

See it as an opportunity to learn an important life lesson: if you don’t make an effort to get the things or the person that you want, they will go to someone else. Next time you get to know and care for someone, tell that person how you feel. Whether he accepts or rejects your affection, at least you get to find out whether it would have worked out. Try to move on from this guy and be prepared for the next opportunity to connect with a guy you like. I also agree with the others that you should put some space between this couple and yourself so you have time to get over your crush.

When I was 14, I was in the same situation. However I quickly got over him when I realized he liked my friend. We aren’t carbon copies of our friends so its easy to use this to get over your jealousy. My friend whom I cared for was a very good person to me however I knew her flaws. She didn’t care to learn new things (a bit of an air head) and looked nothing like me. She didn’t like sports and all she cared about was music and looking pretty.
I on the other hand liked the same sports and loved learning and studying. The boy we both liked shared all my same interest. Then I realized he was looking for the opposite of himself. Since I would never care to change myself for a boy than I had nothing to be jealous of. I simply moved on and wished them well. Funny enough they broke up eventually and didn’t even remain friends nor did she and I. Not because of their relationship but because our differences eventually came between us and he and I remained friends for years later. Not today though. That was ages ago. We eventually grew apart and moved away.