You love A but now you're just blinded by B.
If you really care for B, as soon as you got back from your business trip you would've leave A but your heart says no.
Deep down your hearts know that A is supposed to be with you but you just don't wanna admit it.

Stop playing around with B for now cause you're breaking her heart.
She is really into you cause after you mislead her so you can't blame her for falling for you.
Buy deep down that sense of guilt you have is not for B that you thought initially but actually is for A.

This is your own wrong doing and you have no right to decide what you should do next and create all this drama and prolonged it.

Tell A the truth of what you did.
She should given the option to decide and not you.
If she really willing to accept after what you have done.
Able to supress all that heartache knowing that although she's been so faithful but then you still went around and screwed her behind her back.
If she is still willing to give you a second chance then you know that she is all heart to you.

A should be the obvious choice if she chose to be with you la that is.
Spend more time with A and you will eventually realise why your heart silently chose A more than B.

My 2 cents