"No" is a great word - it is your friend and ally, easily understood and readily accepted. Saying "No" to more work than you can cope with is a professional obligation quite apart from your responsibility to yourself.
Partners are not in fact malign sociopaths out to grind you into dust; they have their own pressures and often don't have capacity themselves to take the time to measure your workload. When people do not know what you have got on your plate they will load it indefinitely - why would they not? You have not asked them to do otherwise!
You are a trained professional and while you might wish to blame others for your workload it is your responsibility to ensure that you can deliver work in a timely fashion and of the required standard.
Saying "no" politely and sensibly when failing to do so would be harmful demonstrates that you are able to manage yourself, aware of the needs of the firm and the client and are able to communicate effectively with others.
Being a "yes man (person)" might make you feel and (for a short time) look like a positive contributor but in the medium term it will adversely affect the quality of your output and your ability to deliver work on time which means letting others down.
Firms, like any employer, have a duty to protect their employees from the adverse effects of excessive stress / workloads but this does not remove your power to exercise control and communicate effectively with your colleagues. Take responsibility, take control, say "no" when it is the right thing to say and you will gain respect, feel good and have a long and healthy career.
Your working life is a marathon and not a sprint. Usain Bolt would swiftly come a cropper if he tried to run 26.2 miles at his 100m speed why should you be any different.
Other juniors lawyers have spoken to us directly about their experiences. Not wishing to be named, one tells us “there seems to be a culture of always being ‘yes’ men which inevitably results in people breaking their backs”. While another reveals he often “stands solemnly and silently in a toilet cubicle” reassuring himself “everything is going to be ok”.