Dating advice

Hi all 

Just seeking a better understanding of how cancer affects relationships .

Ive met an amazing women online whom is from sweden and i am from australia,  from  day 1 she had told myself that she has cancer . For the past 2 years we have texted and  called  each  other  from  time to time . The conversations are very spaced out  between  us as i would assume treatment  would take a toll on her and  her social life. As  i dont know much detail to her cancer  and i try to keep things fun  and enjoyable i find it hard when she  does  not reply to texts or dissapears for days as i tend to worry . Im trying to be more involved in her life but it seems she keeps her distance. We planned on meeting last december over the hollidays  yet we had to re schedule due  to her becoming more ill  and needing time to heal . 

Im looking for some advice as to 

Is it normal for someone going through cancer to not be very social, cancel plans and forget days on which we have planned to spend time together ?

Is there a right approach for me to involve myself to make her feel she can come talk to me about things ?

Some  things i could do to improve our relationship .

Thanks in advance for any suggestions