First, relax. It's normal to want what you are describing. Very normal. But let me be the first to say that if you are worried about cheating and "hate to think about it" then you owe it to yourself and to her to pursue every possible solution to the problem first.
Rango is steering you in the direction of a medical checkup. Good idea. Perhaps there is an systemic reason for her low libido. Can you tell her you understand her lack of drive and ask if she would be willing to see a professional to seek help? Convince her that you want to do everything possible to help HER as well as get your own needs met. She may be struggling with this more than she's telling you she is.
If she is struggling with insecurity and you seek satisfaction elsewhere, when she finds out about it you can kiss your marriage goodbye.
In sum, decide this day what is more important: your marriage or your own needs and desires. Only you can make that choice. If you've already given up on the marriage, then welcome to ECCIE! We can help you here!
If there's a chance for you to help her--and yourself in the process--then go to it. Only you can make the choice. And I know you'll do the right thing.
Good luck and all the best,
JDN