I 31m gay have been getting feeling towards my bestie 27f of over 10 years and don't know to do next

So in contex am 31m gay only ever been with guys since was like 14 am currently in a long term relationship of 8 years but hot few snags last couple years. Anyways always been close to my friend 27f well since we hit it off years ago to me she's only real friend I have that am truly comfortable around and able to share stuff that can't tell anyone else.

Last few months I have been hanging out with her lots more see she has been with this guy on and off for like 5 years complete waste of space absolutely doesn't deserve her but never been my place to try force her to stop seeing him.  Anyways she finally did split again from him few weeks back over his views on certain things she been really down from it all so been spending time trying to cheer her up but gosh something inside me has changed when I see her smile and laugh and be happy I can't help but think to myself she's absolutely stunning.

Been times when I been staying at hers for a night to just keep her company and before we have cuddled watch TV with no problems just friends last time my heart was pounding when she was on my chest and I just felt this rush of weird feelings she was a bit concerned with my heart rate made up lie said probably the heat I will be fine.

But for hours after she just lay there all I could do was look down at her not looking at TV at all my mind racing like wtf you doing then few seconds after that had ask her to move since for first time in life I got a erection around a female. She didn't notice by that time I was acting weird she was questioning me on stuff and if am okie I said yeah just said I have to go home said love u speak tomorrow and left went home.

Have no idea where to go from here for one thing I love my bf yeah been hard few years but love him and second she's my best friend I don't know what to say or do around her right now.

TLDR having feeling for my best friend of over 10 years and am getting confused on how to manage the whole thing.