I'm pretty sure you do. Like 99.9% sure you'll lose all the cash. What's the issue?
If I were you just make a separate profile and start a new game. This is also why I do saves randomly, so I can go back and "start over"
To be honest you make plenty of cash in Single Player so I personally wouldn't care about it. You can't even access all the guns until you get towards the end of the game. By that time you'll have money to spend on them. I guess the only thing I'd see you need the money for is to buy and customize cars.