It sounds like you want to create a company (digital agency) as opposed to a sole proprietorship (freelancer). This doesn't mean that you need to start a huge company, but it does mean that you need more infrastructure. Infrastructure will give you more leverage.

I founded Glass & Marker (www.glassandmarker.com) with 2 creative directors who were tired of being paid low freelance rates, and working at big agencies who stepped on their ideas. Today, we've been in business 4 years, and we have a strong portfolio of tech clients (including over a dozen YC alum companies!), and a much larger team. I helped my partners by building business infrastructure; a strong and profitable vehicle to make content for our clients.

My business partners and I complimented each other because they wanted someone to run the agency who knew how to (1) generate new business (2) negotiate (3) ride the delicate balance of client management and enforcing the rules of the SOW (4) manage employees and (5) identify new areas for growth and scalability (expanding services, hiring, pricing, etc.).

My advice would be to find your business counterpart. Let him or her handle all money, contracts, difficult client scenarios, and all of the other stuff that aren't interested in. You have much better things to do with your time and your talents. In looking for a business partner, I recommend identifying a candidate with these characteristics:

-Transparency/Honesty: Clearly you don't like it when you have a client who is shielding the truth from you. 3x markup on your work is too much, especially if they are getting that while asking you to drive their price down (if you got to name your ideal price, it doesn't matter if they made 10X). Find a partner who speaks truthfully and doesn't mind pulling back the curtain on finances, even in front of client. I do this, and it has always been appreciated.

-Confidence: Your work is valuable. You want someone who understands that, and isn't going to wimp in the negotiation with the client. Sales hungry leaders often short themselves because they can't play chicken with the client. Get someone who will hold their ground in the sale. Ditto on revisions. Not that you want someone who is keen on conflict, but don't get someone who is conflict averse or passive aggressive. When clients get pissed about overages (even though they totally blew past the scope), your partner should be able to resolve conflicts while making the client both feel positive about the work and paying the additional fee.

-Meticulousness: If I let a client get a revision for free, it is definitely not because I'm obligated to. I made sure that my contracts were written simply, and effectively. That gives me confidence that if I had to, I would be entitled to enforce a tough client decision. I have NOT ONCE had to use a contract on a client and my agency has had over 130 engagements to date. I'm also organized with my account management, and I've trained my account managers to be this way. When a client first takes a step outside of bounds, we address it politely, immediately, and clearly. We don't let small grievances build up until the final stages of the project. Organized and meticulous communication is key, and your business partner should be a master of this.

Those are my initial thoughts.

TL;DR: Sounds like you need a business partner. Let him or her handle all money, contracts, difficult client scenarios, and make you more profitable. Find someone who is honest, transparent, confident, and meticulous.