Hi... You are experiencing a typical workplace dilemma. Since you have recently shifted from being a student to an intern and later an employee, you are experiencing a shift in your perceived social expectations with respect to your colleague and your role.
There is nothing wrong with you or your dilemma, it's just that you are overthinking about it as you don't want to offend either side.
I suggest you speak to this elder colleague of yours about your concerns and ask him if he is ok with you calling him by his name? You can share with him that you respect him but also want to keep it professional in a workplace setting.
I suppose if he himself is not a fresher and has spent more than 5-7 years working, he won't mind you calling him by his name or anything as long as you are performing and behaving appropriately as per your role.
Next Steps
Seek professional help (counselling/psychotherapy) if you need further assistance
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