striveforhonesty~
I actually have pretty much walked in your shoes, in that I was also disloyal and was used to "taking care of things" on my own rather than sharing it with someone. Just for some background, my first marriage was an abusive one where opening up meant being hurt, so yeah--I learned to just shut up, cover up, don't open up. What I had to learn was two things:
#1--at first being open really felt like I was being spied on to me. Actually that is kind of natural because the habit I had learned were to keep every thing and kind of "protect myself" from admitting things weren't perfect. So letting someone else in to look at me and what I was doing and who I was doing it with, etc. felt like someone was looking over my shoulder and I never had a moment alone. If this is sort of how it feels to you, just realize that going from fairly self-sufficient/closed to sharing yourself/open just IS going to feel odd. It's okay and that's natural--keep going.
#2--practice telling little truths all the time...and telling some medium sized truths...and then stick your neck out and tell a big truth that is something you kind of don't want to talk about. When I envision "being open" with another person, I envision "being see-through" so that they can see through my exterior and my interior thoughts and feelings are right there for them to see. So I don't TRY to keep it to myself! Next, every day I practice small truths like expressing my real opinion, my real thoughts or feelings but it's something sort of small and easy like what to eat or what to wear. I don't mean "practice nagging" but more like this: you know at the end of the night you're kind of craving mexican and she says "What do you want to eat?" and you think "I'll let her decide so that I'm considering her preference"??? Well instead practice being honest! "I want to be considerate of what you want honey, but to be honest I'm kind of craving mexican!" Every day share a thought that's small but may not be "popular", a feeling that may not be obvious, or something about yourself. Every day share something like "Well this may be kind of messed up but I was afraid you would think..." Just practice that a little every day.
Then every so often try something medium sized, like something that occurred in your past (before her). Or something that occurred at work that made you realize XYZ about how you view relationships. Tell the truth and open up about something that's not HUGE or life-shattering but is a little bit big or maybe a little bit risky. Maybe even speak with her and have an agreement that you'll say "I'm going to open up and tell you the truth about something" and she agrees that when she hears that you're feeling a little vulnerable and she'll respond with "Thank you for being honest. Let me think about what you've said."
Finally, a situation will come up where your gut response might be to avoid it, pretend it didn't happen, "little white lie" so she's not hurt, or you'll just be afraid to tell her the outright truth--like you got a demotion at work or OW sent you a chat and you didn't answer but you're afraid to tell her. THAT is why you have that agreement beforehand with smaller/medium stuff. When this day comes, and you know it is NOT alright for you and you're worried....practice going to her and saying "I'm worried this will go badly but I'm going to tell you the truth because I have faith in you that you can handle it. Here goes. Today I got a chat from the OW at work. I didn't answer it but I did freak out and didn't know what to do. So I'm telling you it occurred and it's still there if you want to see it just to see I'm telling you the truth."
Practice...Practice...Practice!