Envy - lack of Sympathetic Joy. In order to lessen the power of envy, we need to learn how to reflect on our good qualities and the good qualities of others, and learn how to take joy in them. If another person is having joy then our Compassion/Lovingkind wishes are fruiting at that moment. That person is overcoming their suffering. So, we don't want to leave them in suffering, we wish that they will get out. When we see the result we can try to take joy at that moment - that person is not suffering.
Concerning our own happiness in the past, if we think back on when we have felt joy, when we have felt confident, we know it feels good. How would it feel if another person didn't want us to have it at that time? So we will try to practice Sympathetic Joy, taking joy in our own practice and in the good qualities of others. Remembering moments in our past when we touched our potential and the qualities that brought it about helps us develop Sympathetic Joy in our own potential. Rather than thinking about how much we haven't done, it is helpful to remember what we have done.
Sometimes we get lost thinking only about the things we haven't yet achieved. Compassion/Lovingkindness doesn't usually balance the mind at that time because it is not the appropriate unselfish emotion. The appropriate unselfish emotion is Sympathetic Joy with the good qualities of ourselves and others. Sometimes it is helpful to think of other people in the world who don't have a path, and then we will feel more content that we do have a path and a way to develop our mind.
Often we get kind of greedy. It's like we are drowning and a boat arrives and we get on the boat and then we are full of sorrow that the boat does not go more quickly to the land. At least we have a boat! And of course we can always ask that question, who would I rather be, the person I am now or the person I was before any Dhamma teaching or practice? I know what I would prefer. I wasn't capable of helping anybody, myself included, let alone someone else.