Do you love her, want to be with her and fancy her?
Does she love you, want to be with you and fancy you?
Do you have any children or other caring responsibilities?
Do you both work?
This is so complex I don't believe it's as easy as saying you've done everything and now time to give up. I think you need to know whether she wants to be with you romantically/relationship wise aswell as companionship/friend wise.
Why or when did things start to slip? Did it coincide with anything?
I've been in a sexless marriage for around 2-3 yrs, we've worked hard to get this back, and it has come back but do you know what was mainly at the crux of it? I wasn't really enjoying sex, it was boring, I didn't orgasm therefore I didn't want to do it. It wasn't fun. However I never actually realised this until we worked at things, my self esteem increased and my ability to communicate what I wanted etc, but also learning what I actually liked......on my own.....then taking that into the relationship. With my partner respecting this. It's also working out what makes her feel good, what makes her feel sexy and wanted and not just there because you want sex (and I'm sure your not thinking this, but just to put it in straight forward terms!). Not just there for your pleasure. That it's her that you want.
Saying all this she has to be open to recognising how important it is to you, how it erodes your self esteem and feelings of being loved and how that's impacting the relationship. If she's not open to even trying, or doesn't want to then that's a whole different situation.
Do you think she feels like she can talk openly with you without fear of judgement/shaming/embarrassment/you trying to twist things etc?
Some sex therapy would be useful if you could afford it, there will be more to this than you realise.