Feeling hopeless and lost

My husband has ADD, I do not.  With treatment and using reminders, the day-to-day has gotten significantly better. However, the long-term responsibilities are totally beyond him.  After doing the couple's seminar last year, I have made the conscious decision to take a step back and allow him to do what he says he is going to do.  This has mostly failed miserably and is making me feel completely helpless and hopeless. 

The latest example is our car.  We bought a new car almost exactly a year ago.  He did not want to trade in the old car because he knew we could get more by selling it ourselves.  I said if we didn't trade it in, that meant he was 100% in charge of getting the car ready to sell and selling it.  He agreed and said he would clean it out and get it on the used car lot for private sales near his work.  Within weeks nothing happened, except the check engine light came on.  He brought it in and found out what was wrong.  He didn't want to pay the $200 in labor, so said he was going to see if his friend could help him fix it.  About 6 months later, he finally asked his friend and they fixed it one weekend.  He was so proud of himself for fixing it and saving the $200.  During this time we had to pay the registration, smog check, etc.  Could have avoided paying that if it wasn't our car.  The car sat again for a couple more months and the battery died. Then we got a recall notice.  Took him a couple months to call the dealership to get the recall scheduled.  He then paid $200 to get it detailed to sit on the used car lot.  While driving it there, check engine light came on.  This time it was the transmission that was shot and it is going to cost more than the car is worth.  So we have to total it.  If we would have traded it in a year ago, we would have gotten $2000 for it.  Instead he has wasted a couple thousand trying to fix it up, clean it, buy a new battery, etc, over the last year just to have it totaled. 

I literally don't know what to do anymore.  He has wasted so much money on stupid things like this, finances are so tight to begin with.  If I step in, I'm the bitch.  If I let him do whatever, we go into debt.  I'm at such a loss.