Getting over someone you never had!

Advice please! I met a friend at work and became very quite a few years back. Although we no longer work together we have stayed in touch and have become even closer over the years. Although I don't consider myself to be gay I fell for them pretty much straight away but I put it down to just having a crush as they are such a fantastic person. The problem I'm having is that they are really involved in my life. We are best friends, speak all the time spend time together, involved in family stuff. But I'm finding it harder and harder to be around them as I feel like I want more. They are already involved in a serious relationship with someone else. I've always felt like there is a spark between us and the relationship is quite flirty there seems to be a physical interest from them but I think that maybe it's just that they are a flirty person. But obviously it's hell because every time they touch me or make a flirty comment or send a flirty text it makes me read more into it than there actually is. Even if feelings are there from the other side nothing is ever going to happen because they love someone else. But I feel kind of hooked on the little tit bits of attention I get to. It also doesn't help that they absolutely adore me and are literally the best friend I've ever had. It is more than a friendship I know they rely on me too and are so loving and caring towards me I feel like I need this in my life. So I'm caught in a vicious circle of wanting to get over this but finding it difficult to create the space I need because I can't stay away and because I don't want to hurt them by being distant. I know what I need to do but it's hard. Any help would be amazing as this is really holding me back in my life and stopping me from moving on nd finding someone and being happy. Not to mention incredibly painful.