Your accountant is right, and here's why it matters more than just "being organized." When you mix personal and business transactions, you're making it a nightmare for yourself come tax season. You'll be manually sorting through months of coffee runs, personal transfers, and actual business income to figure out what's deductible. If you ever get audited, that messy account makes you look like a red flag - it suggests you're not taking the business seriously, which can invite more scrutiny. You've been doing this for 3 years; at that point you're not in startup territory anymore, you're an established freelancer.
The fee thing is a real concern, but check what you're actually paying. A lot of banks have business accounts with minimal or even no monthly fees if you keep a modest balance or set up direct deposits. Some charge $10-15/month, which is genuinely cheap compared to the headache of mixing everything. You'll also probably save money on tax prep - most accountants charge less to work with clients who have separate accounts because it takes them half the time to do your return.
Beyond taxes, there's the liability angle too. If something goes wrong with a client contract or a dispute, having cleanly separated finances makes it way easier to defend yourself. It's not glamorous, but it's one of those things that takes like an hour to set up and saves you dozens of hours and potential stress later on. Just open one.