Keep in mind that if you have 25% of the stock now, it's very unlikely you'll get 25% of any purchase, if there is one. The buyer will direct most of the deal toward earn-out for employees they want to retain, and try to minimize money that goes to shareholders. It's also easy to dilute minority shareholders over time, e.g., by giving currently employed founders big option packages. Generally speaking, common shares owned by a founder no longer active in the company are seldom worth anything, unless the company is a major success.

On the other hand, given that the company hasn't really succeeded so far, it's not clear that your efforts to date are worth all that much. If the company goes forward, the real value creation is still in its future. So you're not really being screwed over, you're getting fair value for what you've done. The tough part is recognizing that even though you've put in a lot of effort, and invested a lot of emotion, the results so far just aren't worth that much. You sort of recognize that by walking away. But now you want a share of what the other founders create in the future, which isn't fair. You only deserve a piece of what's been created so far, and by your own description, it doesn't sound like that is very much. You can't have it both ways.