How do I adress the lack of sex/intimacy in a relationship? My girlfriend 24F and I 24M have been together now for almost a year now and our sex life has been declining over the past few months

My girlfriend 24F and I 24M have been together now for almost a year now and our sex life has been declining over the past few months. We had a lot of sex at the start of the relationship(basically everytime we met) but over the last 3 months we barely have sex anymore, about once a month, which does not really match my needs.

Of course i respect her boundaries if she doesnt want to get intimate but  I also feel that ‘the lack of sex’ introduced some tension into the relationship from my side.

She often initiated sex at the start of the relationship which was great for me but recently she doesnt anymore and i always get a ‘no’ or pushed away when i try to initiate it. I have also tried to adress this but got kind of ignored. (She just says that she isnt in the mood and cannot provide me with any additional information, so i also dont really know the reason behind it) We are both in our mid twenties so the age should not really be a problem. 

I’m also currently living about an hour away for an internship which was not the case at the start of the relationship, but the frequency of our meetings has not changed because we were both busy anyways. (we see eachother 1-2 times a week)

I need your help or opinions on how you would approach this situation. Of course I want to talk about it with her but I also dont want to be ‘ignored’ another time. 

And my question are: 

Have you ever been in a similar situation? 

And how did you deal with it /approach it if you were in a similar situation? 

How often do other couples have sex?

Is there a way to get her aroused even if she isnt in the mood beforehand?

Are women generally less aroused when being stessed? (because of the busy shedules)