You might approach your Rav with a letter of evaluation from your doctor or psychiatrist, outlining the relevance of your condition to family planning and stating that it would not be wise to embark on a pregnancy in your current condition and circumstances, or you could sign a release to enable your psychiatrist to speak with him directly.
At the moment it sounds like you have permission for the next number of months to continue to use birth control in any case. If you still want a different opinon, you may speak to another rav on condition you inform the second rav that you already asked the first. Ideally the second rav should be someone the first rav respects and would listen to. If you feel that you are in imminent physical or emotional danger, of course, you can and should ask another rav regardless of the first.
You might want to spend the current break that you have been given, working on the issues that are stressing you, either alone or with professional help. Investigate with your physicians the possibility of using other medications that have less teratogenic risk or to further evaluate the risks of pregnancy while on lithium (see for example Cohen LS et al. A re-evaluation of risk of in utero exposure to lithium. JAMA 1994;271:146-50).
If towards the end of the time frame for which you have been given permission, your circumstances are such that you still do not feel ready, you are now asking a new halachic question and can approach another rabbi. When asking the halachic question, be clear about the fact that you do desire to have a family and share with the rabbi your concerns about pregnancy at that time. Come prepared with written information, such as a note from your doctor and or psychiatrist about the dangers of pregnancy at this time and the dangers of pregnancy while on Lithium.
You have our greatest respect for addressing your challenging situation with such honesty and courage.
Note: Click here for more information on pregnancy and bipolar disorder.