It is natural to feel irritated when such things happen. Always try to see the brighter aspect. The task assigned to you is a difficult one, yet you are able to solve them. Unknowingly, you are becoming an expert in your work, but you are not able to enjoy this achievement as you are wasting your time thinking about the junior. Please try not to bring negativity into your life, as this will hinder your growth. In fact, I am sharing this because I know of an individual who faced a similar problem. He was very upset like you, feeling that when all the trainees are paid the same salary, why should he work more while others are enjoying.
I gave him the same advice I give to you. It was not easy, but he tried to do his work with dedication and also helped his team. Today, he is working as a project manager at a well-known company and received personal appreciation from the CEO for taking on the most difficult jobs and his helpful nature. He received over a 100% raise in his salary in just 1 year. So, the choice is yours.
All the best.
Regards,
Anita