It’s obvious you harbored bitter feelings and wanted to make yourself feel better by sending the letter.
So, after this happened, you let the entire thing get to you (very understandable), sat on it for a couple of years, and then took some time to write the letter. It can be argued that this letter is of no consequence other than that it makes you feel better. In sending it, you are admitting that you have felt “bad” over the past couple of years because of what your ex-boss did.
Don’t get me wrong, I’m glad that you have overcome and are now able to experience some measure of success. But since you needed to get back at this person after waiting all this time, it shows that they have exerted influence over you that whole time. So you allowed the balance of power to shift to your ex-boss.
Sending the letter reinforces the fact that you could not get over this initial setback (at the time), and now you have given your ex-boss another reason to remember her “victory” over you. Sure, what she did was screwed up and wrong, but morals often have little to do with power.
One last thing- you have burned a bridge with that letter. You’re probably asking “what the #$@ would I ever need her help for in the future” but to gain power, you have to know how to use your enemies. Make them work for you- don’t empower them by showing weakness and reminding them of previous victories over you.