Jealousy usually comes from wanting something that somebody else has and feeling unworthy, compared to someone else's qualities. We wish to have those qualities, but in a negative way. So we start to have aversion towards the other person, and sometimes there is a wish to pull them down, resenting what they have, wanting to have it for ourselves.
An antidote to jealousy is to develop Sympathetic Joy. Sympathetic Joy with the good qualities and good fortunes of others. We can't usually do this for others until we have done it for ourselves. So if we have a lot of jealousy in our life, it may be that we are not able to see our own good qualities, not to be able to take joy with them, developing the sense of unworthiness, so that when we see the good qualities or good fortune of others, we feel threatened by it, rather than being either equanimous or having happiness with them.
It is often rooted in a sense of underconfidence and Non-contentment with what we have in our life. So by developing the reflection on How Fortunate We Are, we feel more contentment with what we have and the opportunities we have, and we don't feel so threatened by others, materially or mentally. When we see other good qualities, if we have learned how to reflect on our own good qualities more often, than this tendency for jealousy doesn't creep in so much, we don't feel so threatened by another's good fortune or their good qualities.
It may take some time, because learning how to reflect on our own good qualities is something that is not usually encouraged in our societies. What is usually encouraged in our societies is to reflect on what you lack, so that you will seek outwards for sources to make yourself better in some way, physically or mentally, and you become "good consumers."
So Sympathetic Joy, reflecting on your good past Kamma, learning how to feel joy with who you are and your development. Letting go of the excessive idealism that because you are not yet fully enlightened, you are useless and unworthy.
One question I asked you to ask yourself when doubt gets too strong, is who would you rather be, the person you are now or the person before any Dhamma teaching or practice? Learning how to reflect on your growth and the changes that have occurred is often, because moment to moment, week by week we don't see that growth so much. But if we look back a fair while, it's easier to see. It's like a little puppy, we see it when it is born it is so tiny and fluffy and gorgeous. Then we go away and come back six months later, this great big dog is bounding towards us. "Where did the fluffy puppy go?" But the person who is with the fluffy puppy each day, saw all the changes so for them it's not such a big change.
So day by day it's not such a big change, week by week it's not such a big change but over time it's quite a transformation, learning how to take joy in one's growth helps us to lessen jealousy.