Practice will help. Remember, in your lesson you don't have to do anything you're not comfortable with doing, you don't have to go faster than you want and you can stop any time you like. The more you are in the saddle and finding that the horse listens and obeys, the more comfortable and confident you will feel.
To help yourself physically relax, remember to breathe! When we get tense on a horse, we often hold our breath without realising it. If you take a moment or two (either when walking or standing still) to just concentrate on breathing steadily, with your stomach moving rather than your chest, it will help your whole body relax.
You pointed out that the horse is very willing to stop and you need to be firm with him to get him to actually do anything. I love lesson horses <3 They are perfectly happy to stand still for the whole lesson if that's what you want to do. Your horse has seen it all. It doesn't surprise him when someone raises their voice or the rider does something wrong or a car goes past. He's in his home, with many of his friends nearby. So he's not going to spook or bolt.
And, if he did, you're in an arena. He can't go far and he can't build up loads of speed because it's enclosed and he'd have to turn corners, which slows him down. Add on the fact that the instructor is there, and perhaps some other riders and horses - if he did bolt, he'd maybe canter a few strides then slow right down. He would definitely listen to you telling him what to do because he wouldn't be scared running out in the open with the whole world in front of him. He'd be in a place where riders always tell him what to do, where he's quite safe; he can't get so scared or nervous in the arena because it's all so familiar to him.
It will take some time but you've already made the biggest step in the right direction by getting back on a horse. It's okay to spend some time just doing very simple, basic things, reassuring yourself everything is alright. Explain to your instructor that you're feeling nervous and anticipating a bolt or shy, and ask if you could spend your lesson doing simple things to help your confidence. Walk around the arena focusing on your breathing, do halt transitions (asking him to stop) all around the arena so you can feel how easy he is to stop, ride circles so you can see how much he listens to you. As you walk around and think about your breathing, try and relax each group of muscles so that you can feel how the horse is moving. You'll feel that he's moving along quite slowly and lazily, that he's not tense at all. Believe me, even us beginners can feel when the horse is tense and nervous! Have the instructor walk alongside you if you need to. There's no need to feel silly or embarrassed about having lost some confidence.
If you believe everything you read, better not read.