Hi suzzie, I don't know you that well, but I recognize your avatar name. I am just leaving my thoughts to your post.

So, take what you like and leave the rest behind. I always write this because everyone is different and circumstances are different. Plus, I believe people have free will to choose what they like and what they do not like.

It could be helpful if you could clarify the situation. Your boss could acknowledge that you have additional responsibility with this training assignment. You could ask about the responsibility of the work being done. e.g. who is responsible to make sure the work is being done correctly. Just some points to indicate that both you and your boss understand the process involved with completing the two assignments(one, training new employee and two, completing the current amount of workload.) As you have pointed out that more effort and work intensity is required, your boss has acknowledged that with the extra help plus the employee you are training.

Somewhere during the process of clarifying the workload situation, your boss will see your point(s).