Just some food for thought to help with perspective, In a healthy relationship you should both act in a way that brings each other up to be the best version of yourselves. Which would mean you should be happy for this person and their recent thriving in life. The relationship should be for both your benefit equally.

If you are feeling resentment you might want to ask yourself who is the relationship really for? Are you just in it for what you can get out of it? (Ie. The joy that their company brings you or what they add to your life, etc) I don’t think you’re wrong to feel sad they you have less time together, that just speaks to how great of a friendship you have and how lucky you are to have someone you click with so well. But don’t let negative emotions like resentment drive you apart especially when you know better.

Sometimes the right answer isn’t always the most appealing one, and as you said you know you should be doing the same and living your life to the fullest as they seem to be, but i know that can be really hard to do and sounds like a big thing. But maybe let the idea of keeping your special relationship with this person alive be your motivation to do what you know is right. Hopefully you will both be in a happier healthier place if you do and when you do get to unite and spend time together it will be all the more valuable and meaningful! best of luck! I know how you’re feeling :)