You seem to be taking things too seriously.
For the bridesmaid, there's one or two half days when you go for fittings and then the day of the wedding. Apart from that it doesn't require any attention from you at all.
For your boyfriend, you should have told him that it's fine if he goes and you should have told him to have a great time.
If you want to break up with your boyfriend for any reason or no reason at all, you can do it. At any time. On any day.
If you were to break up with him, it would be no big deal. He'd get another girlfriend and you will get another boyfriend.
It's better if you have a policy of not trying to change your boyfriends and not trying to control them. And also at the same time, not allowing them to change you nor control you when it comes to your time, your reactions, your humour.
It is possible to go to Amsterdam and enjoy the tourist attractions, the city in general, the museums and art galleries. Without touching the sex workers nor drugs. Whether your boyfriend is that kind of guy is something for you to judge.
You can't rely on him telling you the truth if he does something with a prostitute or with a tourist that he meets.