You could also join a local mother and baby group that meet even once a week and you may find talking to them and realising some of them are going through the same problems may help you. good luck
Low blood pressure can make you feel light headed . You didn't say if your baby was sleeping through the night .....Lack of sleep can make you feel really really bad and like you are unable to cope . Do you have family or friends that can help you out and give you a break? Are you still breastfeeding? You need to eat extra calories if you are and it is best to eat something or drink some milk after breastfeeding as you can feel lightheaded then too if you don't. I take it you have already been to the doctors about this and they don't know what it is. Try and take your baby for a walk every day if you can and eat properly hopefully you will be back to normal soon .