I like friend’s sister but can’t tell him...

I have a crush on my best friend’s sister and can’t say anything to him because he knows everything about me. He knows I had a bad relationship with my ex and may assume I am just horrible to be with, which makes me sad because I know his sister likes me, too. If he were on my side, he could have put in a word for me and convinced her to go out with me just once, to get a better idea of who I am as a person. I am always sad when I meet him because I know I can’t tell him what’s on my mind. I can’t forget about his sister and don’t know how to deal with this. Should I assume it will never happen and try dating someone else even if it makes me unhappy?
Why are you doing your best friend such a disservice by pre-empting everything he may say or do? If he knows about your former relationship, doesn’t he know who you are and how you behave in general? Is his opinion of you really going to be coloured by that, or by what he thinks of you as a person independent of what you are when you are with someone? If this is on your mind as much as it is, why can’t you be honest and simply let the cards fall where they do? A strong friendship is about openness and the ability to have conversations even if they are difficult. Also, ultimately, neither your feelings nor your friend’s opinion matter, because neither of you knows how his sister feels.

Should I end a relationship because my boyfriend is too possessive and controlling, or try and get him to change his behaviour and be more trusting?
That depends upon whether you have made this attempt before, how serious you both are about this relationship, and whether or not the two of you want to make it work. Why not start by telling him how this affects you?