I sort of sense that you already know how to deal with this so I offer the following advice also for others seeking answers to similar questions.

This is not legal advice and shouldn't be construed as such. You're responsible for what you choose. Remember, revenge is a dish best served cold. You can always reheat it later to savor in your memory, but you don't want your emotions to get in the way of executing your legal revenge masterplan.

I'm going to mostly differ with the cautious, fix-yourself-first, it's-your-problem-of-perspective, advice others are offering. You haven't "let go" of this even after so many years. There is something real there. You're feelings are signaling something to you. If these people treated you unjustly, and you know that, don't cowardly hide from what your own feelings are telling you and hope you can meditate your way out of this. Live. Fight back. Stand up for yourself, and for what's right, and for justice. Instead of putting the blame on yourself to fix your perspective, take action that's legal to get redress for yourself and seek punishment for the ones who hurt you. This sense of grievance has already come to define you. Why dishonestly try to give that up? Lean into that, make a plan, and never ever give up until you win. It might take a while but at least you'll be moving forward. If it is so important, don't "move on" or "let go". Act. To make it better. It's the classic Hamlet question. Moving on or letting go when it really matters to you and when you could act, that's the kind of cowardly weak behavior that perpetrators want you to do so they can get away with hurting you. Your emotions are telling you you are not such a person. You have held onto this for years. That's strong, not weak. Giving up doesn't get justice for anybody. Giving up your fight just supports people hurting others with impunity. So with that said, what steps can you take?

First, try to learn how you can make yourself stronger and smarter so this doesn't happen again in future. Then start your plan.

0. Understand your own goals and feelings. What is the thing you are most angry about? Where do you feel you have no control? Where do you want control? Where do you feel powerless? What do you want to make happen?

1. Sue. Approach some different lawyers. Be patient and explain things very simply and succinctly to them. Don't tell your "sob story", just relate the facts because they're busy and are interested in evidence to make a case that can win, not in the emotion at this stage. A person who treated you like this probably made other mistakes ( in contracts and business ) that can you can exploit legally to win compensation for any injustice done to you. If you can win money from the person, you'll feel better.

2. Thrive. Otherwise known as revenge thrive. Make your own startup. Sell it for more money. Once you're successful, if you still feel aggrieved, you can use some of your money to make this person's life more miserable. Be creative. Start a bad PR campaign. Poach their employees. Fund legal actions against them.

3. Tell the truth about what happened. Name and shame. Make sure your story is known to everybody. Set the record straight to everyone. If this person expects to treat others wrong and be protected by their silence prove them wrong. Maybe you'll encourage others to come forward. Maybe you'll help prevent others experiencing unjust treatment.
The more well known they are, the harder it is for them to claim libel or defamation against you.

4. Adopt some stoic philosophy. Read Marcus Aurelius' Meditations. Today. This will help you sort out which grievances can be naturalized by perspective gymnastics, and which require attention and action.ppp Some quotes for you: "When another blames you or hates you, or people voice similar criticisms, go to their souls, penetrate inside and see what sort of people they are. You will realize that there is no need to be racked with anxiety that they should hold any particular opinion about you.", "Reject your sense of injury and the injury itself disappears.", "If you are distressed by anything external, the pain is not due to the thing itself, but to your estimate of it; and this you have the power to revoke at any moment.", "If someone is able to show me that what I think or do is not right, I will happily change, for I seek the truth, by which no one was ever truly harmed. It is the person who continues in his self-deception and ignorance who is harmed."

5. If you have taken all legal steps of redress, and thrived, and still feel aggrieved, get someone to help you reflect on what is really making you feel angry. It could be a friend, or a professional coach, mentor or therapist. Anger comes from fear. You could start by asking yourself: What am I afraid of?

6. Bonus. If you feel you are protected from negative karma, you can curse them, using whatever powers you believe in.