Medical Director and Professor of Obstetrics and Gynecology
Division Chief, Reproductive Genetics
Feinberg School of Medicine, Northwestern University
Having a purpose for sex beyond pleasure can definitely take the fun out of it! You didn't say if you're having fertility problems, but I'll assume that is part of the issue since you say you've been trying for "months." Generally, it is recommended that you seek specialty care for fertility issues if you have not gotten pregnant after a year of trying.
Infertility, whether or not you are undergoing treatment for it, places tremendous stress on a couple. The longer you go without conceiving, particularly once you and/or your partner begin treatment, the greater the stress. One study of 200 infertile couples found slightly more than half experienced a decline in sexual satisfaction after diagnosis.
I have several suggestions for you. First, take a break from trying to get pregnant. Take two or three months off—that won't make much difference in the long run, and it could do wonders for your marriage. During that time, schedule a vacation. If money is tight, the "vacation" could be to a friend's weekend house or even just a couple of days in a local hotel—anything to get into a new environment with no reminders of fertility issues. Or, have a "staycation"—take time off, stay at home and simply enjoy the time alone with your partner.
Next, try to bring some spontaneity to your sex life. That could mean having sex in a different room, at different times of the day or in different positions. Do not plan sex! Planning is invariably a major part of the problem. In fact, another option is to stop having sex at all for a week or more. Instead, return to the days of dating and kissing. But don't let anything lead to intercourse. You'll be amazed at how turned on you feel after a couple of make-out sessions.