Long Distance Marriage. HELP!

I have been married for 6 years now, but i have not stayed in the same roof with my husband for 1 month due to work.he works in another town from where i am. Is this normal for a marriage to work like this?
 It has been very difficult coping without my husband for that long.though he comes home monthly spent like 3-5 days with us.I have a 4 yrs old daughter,and expecting one. Concerning sex,its very boring,cos we have sex like twice a month(don’t know if this is healthy for marriage) for extra marital affair,I have thought about it in the past,but never did,though very difficult,cos I can be very lonely,and its heart breaking most of the time.
As regard to the way my husband feel about the long distance,he has tried his best(according to him) to get a transfer,cos he works in the northern part of the country that is very volatile,so am even very scared to join him there. So am in a cross road now. Another challenge I have is this,because we do not stay in the same town,he is secretive with his finances. Don’t know what to do. Please I need all the advice I can get.