You might have to consider the possibility that you're poly? you'd hardly be the first. It often comes with a huge amount of shame because society really isn't built for it. That's why many people hide it and either have to fight their crushes or become serial cheaters. I would recommend you read the ethical slut and see if you can get something out of it. There's no guarantee it'll save your relationship but it might teach you some important things about yourself. Either way i wish you good luck. Your situation is pretty hard to deal with.
We have an entire section in our Wiki for books on non-monogamy.
Two that seem relevant and are often recommended on this sub are:
Those might be good places to start.
There are some books you could read such as <em>The Ethical Slut</em> or <em>More Than Two</em> as well as a lot of podcasts and blogs.
Communication is vitally important, you probably know that though :) Needs, desires, and boundaries. And be willing to talk about how you DO feel, even if you think you "shouldn't" feel that way. Time management is important too — make sure nobody feels neglected. I live with my wife, and my girlfriend is 40 miles away, which makes things tricky.
As far as giving advice goes, it'd help to know what specific challenges you've encountered, or what you're worried about. I'd be happy to talk about my experience in those areas.
The situation you're in right now will be impacted a great deal by the things you don't know you don't know. In other words, the questions you don't know you should be asking. In this regard, I highly recommend the two of you read polyamory books together. Three that I would recommend are The Ethical Slut, 8 Things I Wish I'd Known About Polyamory & The Jealousy Workbook. 8 Things is by far the shortest of the three and I would recommend you start there as it's a really great primer for newbies. If after reading that, you're not overwhelmed, you can move on to the other two.
These three books will give you an excellent launching pad for working out what you need from each other in a non monogamous relationship.