How to deal with a guilty feeling after graduating with very high grades as MA student? The question seems quite clear, but here is my situation:

Recently, I have graduated with a MA degree in humanities with the highest academic distinction in my country (Northwest Europe). In fact, as my supervisor told me, the last time that a student in our department reached such a level was a decade ago.

Nevertheless my satisfaction, I feel already that I don't deserve such a distinction because:


I studied for a semester abroad and took 4 courses in a country where A's and A+'s are traditionally "easier" to earn than in my country. I, thus, think that I was in a better position to get this distinction than my classmates who stayed at home.
More importantly, I feel guilty because I once "recycled" a paper from my undergrad years for one of those MA courses abroad, while I only vaguely referred to my previous work. Note that I did certainly not use somebody else's work, but I feel that I could have avoided such an act of self-plagiarism during my MA by referring more clearly and directly. The professor of the MA course did not seem to be bothered with my practice, even when I told him twice that I was planning to recycle an old paper. 


Good to know:

1° My supervisor told me already not to bother with the first issue because I received even better grades for my thesis and for some courses at my home university during the second semester.

2° I would like to start a PhD program in the near future, but I feel that someone will call me out for my current issues or, even worse, I can lose the current distinction/get my degree revoked. 

3° Regarding the paper, I used some major parts of the undergrad paper, rewrote the introduction (adding a literature review) and conclusion. I was also able to publish the paper in a low-tier journal for graduate students (and corrected my mistake by that time.)

How should I deal with my situation? Should I contact the professors in questions and explain the situation? Is this normal to feel like this (aka impostor syndrome) or are these serious issues?

Any advice is helpful!
It is not inherently unethical to "recycle" previous coursework.  It is only unethical to do so without the current professor's knowledge.

Professors may determine, based on their own teaching philosophy and the circumstances of the course, whether or not it is acceptable to reuse work completed for previous courses.  In this case, you told the professor and he consented, so obviously he decided that reusing your previous paper still met the academic standards he set.  Someone else who learned about the matter might have a different opinion, but they would respect the decision he made.

Your actions in this case were completely ethical.  There is no way they would lead to future academic dishonesty charges, let alone a revoked degree.

As for the semester abroad, your transcript will clearly show that those classes were taken at a different institution.  You'll provide those records as part of applying for a PhD, and the professor or committee considering your application will be able to consider them in context, taking into account their own opinion of the standards at that institution.  Again, everyone has complete information, and there is no deceit or unethical behavior.

As for the academic distinction, it seems pretty clear to me: your institution had criteria for awarding the distinction and you met them.  In setting those criteria, they certainly would have taken into account how to consider coursework completed at other institutions.  So again, no deceit.

Anyway, I am not sure about Europe, but in my experience, such honors are pretty meaningless in practical terms.  It will look nice on your diploma, but someone evaluating you for a PhD is not going to look at what distinctions you may or may not have received; they're going to look at what you actually did.  So it's fairly pointless to worry about whether or not you "deserved" something that, in reality, doesn't convey any practical benefit.