I am needed advice from managers!

Last Monday I was notified that I’d begin on another project- on top of already having projects to work on. I am new - have been with the company 1yr and 5 months. There was a change in management and many tasks/responsibilities have been realign. As with other small companies, our company is having a hard time keeping afloat. Many of our benefits, matching 401k, bonuses,raised, etc are replaced with longer hours ( we now work remotely- duration to be determined), limiting outsourcing ( we now have to do the jobs). As a small staff (10) we rely on each other or ourselves for many things. Often times waiting on someone ( who is known to be slow w response) or with others ( like myself) who tends to want to get things done. Being fairly new has been interesting especially because there has been a lot to learn- all good! I have had my ups and down- especially with a co-worker who is precise and quick to point errors. Again, it’s good learning. However, this same individual ( before new hires) was tasked to do what I am now doing. They eventually ( after training me) moved on to other projects. In the when being trained by them, I had various issues- they were in essence drowning with tasks and in need for me to learn as quick as possible to relieve them. This was never conveyed to me- I came to find this out later! Naturally I tried to pick up but made mistakes. Management got involved and volunteered they started on the wrong foot without a plan to train me- they just expected me to pick up. All was resolved with plan and weekly check in’s. A year later - after being thrown outsourced tasks I am now tasked to work as backups in other projects along with being responsible for their sub- tasks- following up on publication, sending emails, etc. In the last few weeks I have been working long hours, 4 hrs past my clock out time, stressed, unable to enjoy my fiancé or home life. I am trying to breath and finish strong but I am making mistakes down the line. A supervisor spoke to me the other day and he said that I need to slow down. Take a breath and re-read, formulate my strategies for email and so forth —-and not rush into things- just his advice, he added. Now, this same supervisor is the one who has me working on side tasks every week: research this, follow on this , etc. Worth to say that he has also the reputation of taking his time with tasks and people are always waiting on him. In the past other staff members questioned if he was shifting his work over to the co-worker who trained me. Long story short: I need advice! I would love to hear how to handle this- when to push back and when is a good time to speak to my boss? How can I express to them that I am at my limit without them coming back with : well, didn’t so so have this before? They did it with other things as well. Or they bring up my mistakes Please help