How do I stay true to my principles even when it costs me?

You have set your life so that your word and your work say the same thing, and you would rather forgo a gain than buy it with your integrity. That is a rare wholeness, and it is worth more than any deal it might cost you. The cost is real, and you should not pretend otherwise. There will be money, promotions, and easy exits you turn down because taking them would mean becoming someone you do not want to be. But look at what you keep: a clear conscience, an unclouded name, and the simple peace of never having to remember which version of the truth you told whom. When the test comes, and it will, do not weigh it as a spreadsheet. Ask only whether you could still respect the person who said yes. Let that be enough. A gain that costs your integrity was never really a gain. It was a trade, and you would be the one traded away.