Everything I do is motivated by greed/aversion. How to react?

It can be useful to think about your actions. Everything you do is an action. Even inaction is a type of action. In other words, you can not escape constantly acting

Actions can be an object that can be meditated upon. You can observe beginning, middle and end of actions.

Attachment or aversion arise if you consider it “your” action. In other words, if there is “You” in that experience - this is mine - This action is mine. But is it your action?

Before we act, we may think and choose. Buddhism considers thoughts as mental objects that not “yours” but what about choice

Choice is just a thought that occurs more predominantly than other thoughts, since none of the thoughts are yours, so choice is also not yours. Since choice is not yours and you are not the chooser of actions, you are also not the doer

"The question is how should I replace these bad motivations by more skillful ones ?"

Excellent question. You can replace harmful neurotic emotions such as greed, anger, and jealousy, through recognizing the true nature of your mind and emotions. Enlightened beings and realized masters have given us a path, with advice and practices on exactly how to do this. This is the main goal of Buddhist meditation practices.

There are many different approaches within Buddhism, depending on one's needs and proclivities. For example, in some practices you would tend to run away from harmful emotions, to seek a kind of quiet peace. In others, you may directly confront them, staring at them directly until you see their true nature, which is actually primordial wisdom. Developing compassion is also the main point of all Buddhist practice.

For example, when understood very clearly, desire becomes bliss and emptiness..and a wisdom of discrimination..

just out of the bat - study hard can equal getting a good job than you can donate a lot of money to people .... you can do better work in whatever occupation you choice and help people more

do you meditate etc ? do you have a teacher ? i think the more you advance in the path your motives will be purer and less about the unwholesome and more about the wholesome

without the trouble of explaining myself i would like to join you with the idea that drooping this things will make things worse

edit : for example if you practice more and you see the 3 characteristics more clearer than your greed will lower automatically and the reason you do stuff for greed will lower but the good reasons will stay - for example you learn to help students and also to show off your smart - with practice you will lose interest in the show off but will still have the will to help others which can even further increase with the added practice of metta

One may find as meditation practices continue, outlook, views, and motivations can change as well. This is encouraging. What was once seen as a feed for the ego becomes food for compassion. Metta meditation practice is a great tool if one encounters ego.

The teachings of Thanissaro Bhikkhu on Aversion, Greed, Ego, and many other topics are accessible and highly valuable. YouTube has many of his short talks on vast dhammic topics and they are available elsewhere. Please invest a short amount of your time on them.

All the best my friend. We are in this together.

Here is a slightly narrower answer.

The three examples you gave (i.e. "be top of your class", "impress people with your piano playing", and "work hard to be admired") are, I think, more specifically, examples of "conceit" or "pride".

See this topic for further details: How are 'conceit' and 'identity-view' not the same? -- the answers to that topic suggest that any form of comparing yourself to others is a form of conceit.

There is, though, a sutta (the Bhikkhuni Sutta) which suggests that conceit can be useful or helpful:

This body comes into being through conceit. And yet it is by relying on conceit that conceit is to be abandoned.' Thus was it said. And in reference to what was it said? There is the case, sister, where a monk hears, 'The monk named such-and-such, they say, through the ending of the fermentations, has entered & remains in the fermentation-free awareness-release & discernment-release, having known & realized them for himself in the here & now.' The thought occurs to him, 'The monk named such-and-such, they say, through the ending of the fermentations, has entered & remains in the fermentation-free awareness-release & discernment-release, having known & realized them for himself in the here & now. Then why not me?' Then he eventually abandons conceit, having relied on conceit. 'This body comes into being through conceit. And yet it is by relying on conceit that conceit is to be abandoned.' Thus was it said, and in reference to this was it said.

So, for example, thinking that, "My teacher plays the piano better than I do -- if I practice like they did, I too can learn to play as well as they do" is a constructive, insightful form of conceit.

In summary, motives like "I want to be good" and "I want to be good (or comparable to) like they are" can be an effective motive.

What you find that motive satisfactory is a different topic; as is what you're motivated to practice; and what you consider good; and who you want to learn from, to emulate and/or to impress.

The question is how should I replace these bad motivations by more skillful ones?

So one answer is that the motivation itself isn't unskillful -- it may be effective. What may be more questionable isn't the motivation itself, but what you're motivated to do, who you're motivated to impress.

Apart from that, some of the canonical Buddhist social motivations are called the Brahmaviharas:

These four attitudes are said to be excellent or sublime because they are the right or ideal way of conduct towards living beings (sattesu samma patipatti). They provide, in fact, the answer to all situations arising from social contact.

One of these, mudita, is I think especially or at least partly intended to alleviate jealousy and competition -- if someone else is good or skillful then be happy for them.