I can see where thus change is a bigger issue than expected. If I follow correctly, you are 26 and have spent your entire adult life with this one woman. I suspect that before now you were trying to live up to the mono-programmed "ideal" of having all your needs met but Just One Person <tm>. As such, having your conduit to the rest if the world close own 2 or 3 nights a week with no alternate is bound to cause pressure.
No, I wouldn't suggest finding a partner if your own. You just might go awry with that. Later maybe. For now, you need to learn how to be you without you being a plural you. In all likelihood, at 17 you were living with parents. You go from mom and dad (or some version if that) to adult life with this girl there. I don't see any gap where you had to be comfortable with yourself with no one around.
Get out of the house. Go do guy things be they sporty or geeky. Go hit on some women (be they sporty it geeky ;-P). Keep yourself busy when she is busy, you won't be beating yourself up so much, and you'll have more to share when you are together.
My mono and I put a lot of effort into making sure she feels appreciated and that our time together is managed well, but beyond that, she is also perfectly capable of being independent and on her own. It's time for you to develop those same skills. It's better to be an independent person who chooses to be with your woman than a dependent person feeling trapped.