You're pretty funny. It's managements job to set the bar so you can "Barely keep up" That's how they extract the most work out of you. If you're stressed out and barely able to keep up, then YOU are the problem. Stop trying so hard. Back the F up. Prioritize your projects, put in your 40 to 50 hrs and go home. If they don't like it they can fire you and retrain someone else. Or maybe they will get you some help.
The fact of the matter is, if you're over-worked and ok with that, they have no business reason to get you help. You are never going to "Talk them into it" because, to be frank, that's not your job. There may be financial reasons they can't hire anyone else, they might be incompetent. Who knows. If you aren't ok with how things are, change what you can. Mostly likely the only changes you can make are in your own behavior and attitude. If that doesn't help, or they get pissed, it's time to move on. There are those in the company that make strategic decisions. You are not one of those people, get over it. You're like the guy at the back of the buss complaining about how the drivers operating it. Don't complain, just get off.