Hey surfmonkey, wow that's a doozy of dilemma. It's a shame your instructor is so discouraging. Their job is difficult, as it's up to them to weed out the drivers who aren't ready to have their license yet. But they should never aim to make a fool out of someone. How discouraging. It should be their responsibility to help you, also. What good would ridiculing someone do?
I know it's easier said than done, but just try and not let him get to you. Driving can be very mental. If you're constantly worried and panicking, it'll be hard to truly focus and concentrate. My advice is to continue practicing with your husband. Perhaps more experience out on the open road will help you feel more comfortable. Is there anyone else that can take you out driving - a friend or parent? It might help to get another perspective from them and maybe they'll be able to give you some tips, too. Don't let this guy get to you. Nobody is going to believe you're a good driver until you do!
Driving, like everything else, will get better with practice, practice, practice. When I got my license at age 16, I also had trouble switching lanes. There were countless times I'd almost run someone off the road trying to merge over. It drove my dad crazy! (pun intended)
I think the secret to merging lanes is awareness. You need to know what's around you at all times. Let's say you're driving down the interstate in the far left lane, and you know that in two miles, you'll need to exit from the right lane. You need to be consciously thinking about merging and paying attention to who's all around you. The worst thing you can do is wait until the last minute to try and merge. It's just one of those things you need to focus on and give yourself extra time to do. Next time you're out with your husband practicing, try that method. Always be checking your mirrors, and if you see cars coming up on your behind, just monitor them. Always know where they are so you'll be ready to act accordingly. You can be the best driver in the world, but you still need to protect yourself from the other guy.