Your boyfriend sounds a decent sort. Worth hanging onto, anyway.

I am wondering which version of stand up for yourself he means though - there are two.

The first version is where you are a total mouse. Never have an idea of your own, agree always with his, or others, opinions and generally get walked on by everyone. This is down to shyness, bad self image, or excessive politeness.

The second is like the first, except you spend most of your time complaining to him afterwards that "If I had been asked ..." things would have been much different. This one is annoying as hell.

Now, you sound more like the first type - your use of the word scares shows that - and fortunately that is easy to start, especially with some support.
Pick where you go when you go out next. Just straight up tell him "I want to go here, tonight." If he suggests somewhere else, say "No, I really want to do what I said." He'll give in or offer to compromise. Stick to your guns. First time there should be no compromise - you have got your courage up, use it.

When you are talking and he says something you don't agree with, disagree with him. Politely, of course. Don't get upset and offended, and he won't. You'll find your relationship goes to a whole new level when you can have interesting conversations with each other. Get some practice in - as you are a member here, comment on some of the answers and blogs you disagree with and see what happens. I say here, as the commenters are polite enough with each other, if not the guys!

Keep doing that and you'll get the habit of being more independent. It is fun, trust me!