Why this happens and what to do about it.
It is probably because you fear letting yourself down. Perhaps you are unable to handle anxiety caused by changes in your life. It is the moment when your brain keeps revolving around imaginations, such as, self-sabotage, lack of the desire to move on in life, or getting potential growth. Even research links the anxiety coming during happiness moments to the fear of expecting negative happenings. Another reason you may be anxious is because of exhaustion from the mental, emotional, or physical energy associated with the good vibes. You could be expending more energy to achieve your goal, which may be taxing to your mental and general health. But I recommend the tips below for coping;
While things may be good on your side, finding help when anxious is a plus. You may be misunderstood, but be sure friends who want you to succeed will stop assuming and come to your rescue.
Find a therapist
Seeking professional help can also help fix your issue. They may offer tools that work through your anxiety and help get the validation required within yourself.
Good life experiences may be stressing because of added responsibilities. But I recommend giving the best environment for your body to feel supported through such moments. Dwell on good habits, including change of diet and exercise for the best health.