ajasue wrote:
No, it's that once we start, we think if other things and get distracted. Once that happens we have to stop because we're not in the mood any more. It doesn't happen every time, but it does happen more often than not

"Don't think about an elephant". What are you thinking about? An elephant? You probably heard this before, that learning relaxation and meditation is the opposite of concentrating. You need to let go of your own focus. You also seem to have turned something that should be pleasurable and loving into a household chore.
Perhaps you need to establish some patterns that suit you both, like ensuring a relaxed environment - wine & dine first, like dating. Be sure to keep the home tidy and clean so that there is no mess or unfinished chores to distract your minds. If you usually have sex in the bedroom, then get rid of all work-related books, the television and all hobby or personal interest clutter from the bedroom.
On the one hand you might want to avoid having sex unless both of you are in the mood, to eliminate the chore feelings. On the other, you might want to agree with each other that you are open to quick, spontaneous sex when one of you feels the urge, wherever you are and whatever you are doing at the time (so long as it is safe and private). I assume that you have both enjoyed sex together, and you need to hold on to those times when it is a mutual pleasure. Talking during sex, and about the things you find exciting, can really increase intimacy.
To state the obvious, you also both need to know about fertility - having penetrative sex to completion on the most fertile days of your monthly cycle - but not to the point that "having the baby" makes sex such an effort that it is no longer any fun.