Here are some thoughts.
1. Throttle back
Think about how long you have been doing the load by yourself. If it has been more than a couple of months keeping that pace, I think you can keep it up for the next 30 years because that is the level you work at. If that load is getting off on time every night since the help left, then management may think "millerlight" can handle it, even if he does complain from time to time. If the pace is too much for you, it is time to slow down to the point that you are working steady. Not fast, not slow, but steady. You will probably catch flak, especially if your steady work rate is much less than the hectic rate you are at now. The day that that hot load does not get off on time with only you working it is the day that you need to clearly let your boss know you are concerned about being able to safely get said load off at said time. Working yourself harder than you are able to physically or psychologically risks injury. If management does not understand that, and instead writes you up for production, take heart in the fact that it would be very difficult for the company to fire you for missing a production level much higher than the level you were informed about and expected to reach when you were hired. If they write you up or start harassing you about the matter, file a relevant safety, discipline, or harassment grievance as often as you are able to. Be professional, polite and calm when dealing with management. Let your steward know about how you feel about working to the point of being unsafe. I would bet you will have a helper or be moved to another part of the building by then end of the week. You need to stand up for yourself and take the small risk that you will be fired, and the slightly larger risk that management will harass you about this matter. Talk to HR, talk to the stewards in your hub, talk to your co-workers about it. If you are not willing to take that risk, you must find a way to live with the consequences.
2. ****e does happen
Not having help time to time during the year while working for UPS is an unavoidable and universal experience. Simple nature of the work. The people that are available to help you move on from UPS, although there is an endless supply of new hires to replace them. Directive comes down from on high that overtime is to be suspended. Never lasts more than a season at a big building, but does happen. Maybe in a month another double shifter will come along. But who really knows when that will happen. Just work steady, not fast.
3. Communication breakdown
If your pt or ft sups know how you feel, you would not find yourself in this situation. Make yourself clear to them. Do this by telling them at least once a day, and more often if you can. Let them know you are upset about not having help. Every day make sure that message is put across to them. Say it in different ways to them, even if you sound like you are repeating yourself. Be polite and professional and friendly when you do it, but do it until you are sure that they are putting it off.
4. And just for the hell of it
Freak out man. Get out of the truck an hour in, go up to your sup and in his face tell he/she, “I can’t take loading this truck this EXPLETIVE fast every EXPLETIVE night. I am not going to EXPLETIVE do it anymore.” Make a scene man. Shout it out, pound your chest. Tell them if they want hot load off by certain time have some newbie loader do it. Hey maybe not your style, but I work in a large hub, I have seen it work, I have done it myself once and it works.