Sounds like a nursing strike. I'm guessing, with all your experience, that you've heard of that( if not, see website below). Congrats, by the way, on doing so well with the older kiddos!
You can get through this - you just need good support, good information, and maybe inspiration. Even adoptive mothers can breastfeed, and you've got more of a jumpstart than they have. Best one-stop spot for information:
www.kellymom.com.
I had problems when my first was around 4 months. I got sick, was very dehydrated, and when I talked to a lactation consultant, she said that the post-pregnancy hormones were leveling out, going back to normal, so relactating would require a little more effort. I'll give you the advice she gave me:
eat more rich foods especially , nuts, avocados, & oatmeal. She also recommended taking fenugreek capsules. (I cleared this with my doctor). Fenugreek is VERY safe, in fact it's used as an artificial maple syrup flavoring. I took 6-7 capsules at a time, usually 2 times a day. (Sounds like a ton, I know). My breath and sweat would start smelling like pancakes. This worked FAST for me.
Also, keep trying to get your baby to nurse. (It was the Friday before a 3-day weekend and she said to "wear that baby like a bra" - or nurse as much as possible all 3 days. And when the baby slept, pump. (I've found that the ISIS manual pump - about $45-50 new - has PLENTY of suction.) Nothing compares to a real baby, though.
Check out your local resources like La Leche League - these women work as volunteers and they are pretty passionate. If you tell them you've already nursed 2 kids past 12 months and want to continue with your 7 month old, I'm sure they'll want to help.
Last ditch - (only because it's not free), if you're in Atlanta, Dr. Kute of Chattahoochee Pediatrics, is also a lactation consultant. (I've had luck with 2 lactation consultants at Northside Hospital: Sandy and Dee Dee. Some of the LC's aren't as helpful, but those 2 are great!)
For "inspiration", read this woman's story: http://www.drjaygordon.com/development/bf/relact.asp
I also work with a man whose wife had a STROKE after her baby was born. She had to go to the ICU while her husband stayed with the baby. Then she satyed in the hospital for a couple of weeks when the baby went home with dad. It was MONTHS before she was able to talk again, and somehow, through all that, she managed to nurse her baby. THAT'S inspiration to me!