I think that you owe it to both yourself and your bf to end that relationship. You should not get engaged to someone unless you are really, truly, madly, deeply IN love them. If there is someone else who you are more interested in, the you should be with them. Simple as. End of.
You don’t want to wake up in 20 years time and fine you’ve spent your life with someone who you don’t truly love and when you think that there is someone else out there who is better for you.
Besides, how good is your relationship with your bf if you’re cheating on him with your best friend. End your current relationship now before things get really complicated, and start a new one with your best friend. It may very well be the best change you ever make!
Good luck!