I think once you begin your studies it may possibly drive a bigger wedge between you and A. You will make a new social circle, nights out, plus nurses work darn hard.
I can see a small parallel from my most significant relationship to date. There wasn't such a big age gap but we settled down when I still had high hopes to travel, spread my wings and attend university as I am intelligent. He would say "do you think I am holding you back" but was so possessive, and could be controlling and insecure.
I got ill so that dashed my university hopes at 18 if you are wondering. He too would take things personally, always had his guard up. If I pointed something out, he would hit back at me worse trying to score points. I wouldn't say he was sensitive, just "touchy."
He hadn't had a serious relationship before, so was quite immature. He would ignore me and take me for granted. If a boy at work said something he would speak about it and I would be like I told you that last week.
Every second word was a swear word and I curse a lot, I am Scottish after all but he would plainly deny it.
At least he agreed to still be friends, you could try that and if there's still feelings well...maybe take a break. You are my age and I still have plenty partying and fun left in the tank!!
I left him as I hadn't lived my life and was drowning in regret. I would never have been well mentally with him. You need more than what he seems to be offering but you see he is a nice guy. It has maybe ran it's course.
My ex thought nothing was wrong when we hadn't slept together for over three months. None of us wanted the next stage - marriage and kids. It was hitting the rocks.
All he said when I broke it off once and for all was "thank f%%k we never booked that holiday"