Read over what you just wrote again. You’ve made it pretty clear that you’ve made effort in many areas to make this thing work. Sometimes you failed, sometimes you succeeded. You seem totally aware of whats happening, yet you don’t want to believe the inevitable: Its time to move on.

The sex disappearing isn’t anything new to a relationship. There are a ton of factors that can contribute, a lot of which you can’t really control. Could you have done a better job of keeping things fresh in the bedroom? Probably. Has she? Probably not. Door swings both ways.

It seems she’s constantly blaming you and not really thinking about her share of the responsibility. A relationship is mutually benefiting…but regardless of who’s doing more, the balance is off and its been struggling to reach what was once “good times.”

That last outburst by her should be the final straw. You wanted to talk about it, she didn’t, then threw it on your lap…with no car. Not to mentioned she was probably directing her words with a lot of "You need to-" "You aren't doing-"...and they usually end in something like "for me" "towards me" "without me."

This is not only being irrational, its devoid of any respect for you after years of dating. Regardless of the situation (and whether you've done this to her or not in the past), it’s a major red flag, and something you now have to act on.

It’s time to end it man. Its always hard, especially when you’ve been together for that long. But when your hand is being forced, like in this case, you have to think about yourself and say FUCK IT.

“I’ve thought long and hard and I’ve realized I am not happy, and neither are you. That means its time to go in a different direction, for both of us. I value everything we have, and I have no regrets about our relationship. Hope things work out for the best.”
Just get something like that out there and get out. I’m willing to bet she comes crawling back, saying “SO YOU ARE JUST GOING TO THROW IT ALL AWAY?!” Don’t entertain any of the BS she is going to throw back at you. Take it in stride, be calm and be the adult (as hard as it may be at this point).

As far as how to keep the sex fresh, there are many places to find advice on what women like in terms of sexual tension, because keeping things fresh, in many ways, is keeping that tension.

A few tips I've seen to create/re-create a sexual frame of mind:

1. "Foreplay" is not just for the bedroom. Foreplay can start early in the day with suggestive texts or pictures. This is key to framing her mind and making her think about fucking your brains out...all day.

2. Switch it up. Switching something like location can be a very simple aphrodisiac. A new setting, visually, will spark something your 1-bedroom apartment won't. How about a hotel room for a night?

3. Talk more. You probably don't talk about sex a lot. The beauty of thinking about shit, is that it manifests into your imaginations. When she tells you she likes being choked a little bit, you have no choice but to think, "YOU JUST WAIT TIL TONIGHT WOMAN." Sometimes, after a long relationship, you stop talking about stuff like this...but it doesnt mean she doesnt want to.

My advice, hit the nail directly. Tell her you think she suspects you of cheating or not being alone because you haven't had sex much lately. Tell her you'd really rather just be fucking the shit out of her. I won't address making the sex good because that's been covered already.