If you are already in a relationship, then going beyond someone's limits is not your normal type, have you convinced yourself that he is not worthy of you or is he not what you thought of him.
Are you one of those people who have been attracted to the story of Romeo Juliet who loves complicated, if so then you should reevaluate your feelings.
Maybe you are always stuck in a relationship with the wrong people, so you should stay away from all this for some time and focus on yourself.
It is easy to develop feelings for someone you spend a lot of time with you can definitely limit any time spent with him but you may not be able to avoid him completely who is not focused on work.
You should stop interacting with him completely and go for a walk with your best friend's ex and as long as you are with your crush he is out of bounds then you should cut off contact until you leave him.
If you express your feelings honestly there is no guarantee that all will go well and in the process you can hurt yourself, so be honest and cautious.
If everything goes well, how do you take it with your crush you can still lose your loved ones and your reputation although all this can prove to be true for true love.
When you are emotionally over invested it sometimes becomes difficult to see things clearly so talk to someone who can give you hard facts.
You have to find a way to keep it platonic because you have no choice but to see it sometimes flirting with anyone openly is a bad idea if someone does it is responsible for it yourself not you.
Whenever you dream daydreams about your crush try to engage your mind in something else like reading books, walking around etc. Just do not think of any person who will never belong to you.
The best way to get someone is to submit to another so keep yourself open to other people too if you do not want to let a great person go away from you then you should start dating continuously.