Hi there! I am new here. But I shall attempt to answer your question. I am chinese (singaporean).
难过 = sad (adjective); literally: difficult to pass/go through (adverb + verb);
艰难 = having a difficult time (at a great degree); has emotional + physical (due to 艰) connotation
艰辛 = having a difficult time (at a great degree); has emotional + physical + tiredness/weary (due to 辛) connotation
To express "I am having a really hard time recently" :-
With "最近我日子过的非常难过", a more correct saying is "最近我的日子非常难过" The "过" in "难过" makes it double as a verb taken literally as difficult (难) to pass/go through (过). So the former "过" is redundant in your original sentence. Secondly, in your original sentence, it should be 得 instead of 的.
"最近我的生活很难过." = here, "难过" is used as an adverb (难) + verb (过) instead of as an adjective (难过).
"最近我的生活很艰难." = here, it is OK, but it begs the question "how is it 艰难?" or "what about 生活 is 艰难?". Although it can be implied that it is the passing (过) of 生活. So you could make it clearer by saying "最近我的生活过得很艰难."
"3:最近我的生活很艰辛." = this is also ok, but it has the nuance that you mean physically. Also, a clearer saying is "最近我的生活很过得艰辛."
Finally,
"对不起。现在我在这里很少, 只是因为在过去的几天里我的生活非常难过"
My suggestion:
"对不起. 我现在很少在这里, 这是因为在过去的几天里我的日子非常难过"
Explanation:
"我现在很少在这里" = This is just grammar... you would still be understood if u said the former.
"这是因为.." = This is a spelling/vocab mistake on your part i guess..
"我的日子非常难过" = This is because the context is about "last few days 过去的几天里" hence i think it is more natural to say "日子" instead of "生活". If instead it was "years 过去的几年里", then "生活" would sound natural. But this part is my own opinion.
With your "只是因为在过去的几天里我非常难过", this would translate to "only because i was very sad during the last few days".