Mentor added his name as the author and changed the series of authors into alphabetical order, effectively putting my name at the last
Speaking from a field where author lists are always alphabetical: This will do you good in the long run! If you follow this convention, you will never have to argue over authorship order - take a look at the amount of questions on this site dealing with exactly this question, and be thrilled that you have the option to simply exclude that from your life.
Since I am in such a field, I also know what people do on their CV: they explain what they did for the study. You can write that you did the experiment, the analysis and wrote the first draft of the paper. This will be more informative for a hiring committee than any amount or author-list ordering principle you can come up with.