We agree with you that it can be stressful for a couple to be given a heter for a relatively short period of time and told to return afterwards and ask again. However, the mitzvah of pru u'rvu and the seriousness of the use of contraception should not be taken lightly, and it is for this reason that rabbis often require a couple to be in touch regularly.
There are many rabbis who will indeed give a couple general guidelines regarding the halachot of contraception (in what situations it may be used, what types are allowed/preferred) and let the couple decide when and how to apply these guidelines.
However, not all rabbis operate in this manner (particularly in charedi or chasidic circles). Many give a specific ruling and tell the couple to return in six months/a year, etc. If a woman is feeling very overwhelmed by a ruling she receives, she can and should express these feelings to the rabbi at the time. She can explain that it is very stressful for her to have to ask again in six months, and request a longer heter. It is for this reason that the woman should be encouraged to talk to the rabbi and not have her husband ask. The husband might not always be able to fully relay the extent of his wife's physical or emotional condition.
In general, if a woman feels she absolutely cannot manage another pregnancy (and gives that impression to her rabbi), she will usually receive a heter to use contraception. However, in some cases the rabbi will still deny a heter. We feel that only rabbis with thorough knowledge, experience and compassion should be giving strict rulings on this topic. Women should also be taught that if a woman receives a ruling she feels she cannot accept, she should either return to her rabbi explaining her difficulty with the ruling, or turn to another rabbi for guidance (she must explain the initial ruling she received). A woman is allowed to ask around for recommendations of rabbis who are compassionate and understanding on this topic.
Rabbis are here to guide us and help us properly observe halacha. If a couple feel that their rabbi is exerting undue pressure on them, it may be appropriate for them to look for another rabbi.
Please feel free to get back to us with any further questions.