I find it hard to accept that God is making such a demand of me, and I feel that situations such as mine simply do not fit neatly into moral theology textbooks
Well it’s reasonable to think that considering your situation, however there are many difficult situations in life that may not “fit neatly into moral theology textbooks”. That doesn’t mean that the moral theology is wrong or incomplete.
Sexual intimacy is reserved for marriage. That is the proper context for it. Nobody outside of the married state should be engaging in it because it is only a good within marriage. Sure…it may be a pleasurable activity, but it is more than that and carries a deeper meaning.
It may be very difficult to accept that this is the case, but God does require abstinence in this case.
It is easy to think that we constitute an exception to the rules, but generally we don’t.
The other aspect of this is the modern idea that sex is the highest good and if we’re not allowed to be in a sexual relationship we are being denied something. If you are married, and your spouse has abandoned you, your vocation is still to honour the vows of the marriage.
If my wife up and left me tomorrow for some reason, I would have no choice but to remain abstinent. I’m reasonably sure I wouldn’t qualify for an annulment and my marriage is valid. I can’t pretend otherwise and the fact is that my vocation to marriage still would stand. My duty would be to pray for my wife and remain chaste in the hope that someday the marriage would be renewed.
It may be difficult, but God gives the strength to do that.