Ran a IT reseller company with an ex-colleague in my mid-20s. We started off and friends but things got sour towards the end, he still hold me responsible for a bunch of things even after I left and threatened to take me to small claims court over $500. There were some highs but the stress of keeping the lights on, getting staff paid, trying to close a sale with only $50 in your bank account was stressful and horrible for my health, I think I dropped down to 130lbs from stress. When things were good, we basked in the glory together, but when things were shit it was my fault. I eventually learned that he viewed me as an employee and not an equal as a business partner.
I stepped away from the business after 2 years and sold off my shares to him for my initial invest of $10k. I should have asked for more, I went 6 months at the start without taking a pay check and he was still getting residuals from the sales I closed. But at that time I was so burnt out from being a business owner I took the crap deal and went back to corporate life.
The biggest take away was learning about myself and limitations. In my early 20's I was open to anything and thought I could take on any role in the company, be it admin, accounts, IT, sales, post-sales and HR. Having done all these roles allowed me to see what parts I liked (pre-sales) and which parts I hated (HR, cold calling).
Anyone looking to start a side hustle should read the book "The Lean Startup". Keep things small and lean until it's the right time to scale up. My ex-business partner and I made the mistake of getting an office space and hiring staff before we were ready.