Too good to leave, too bad to stay

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Hey /adv/, this'll be drawn out so be prepared.
I've been with my fiance for 3 years, engaged for one. I'm 21 years old as of last month.
We began in a long distance relationship, her on the west coast, myself in the east. We hit it off great. Same interests, humor, tastes. Eventually, we broke up for a few months while I dated someone else. This broke her heart, and she tried to get me back by losing weight. Starved herself. I ended up going back to get because I felt as though she loved me quite a bit to suffer so badly. She swore she never would be with anyone else, and that too played part in my decision. I couldn't stand the idea of her being alone forever. I should mention that I was get first everything.
So, here we are, two years later. Engaged and living together.
I'm going into the airforce soon, and she's supported me in this, as she has all other things. However, I find myself daydreaming about other women, I find myself dissatisfied. She's not pleasing visually, and despite her sweet, loving nature, I'm discontent and bored overall.
I don't know what I should do. If I left, I'd feel so guilty over dragging her across the country, as well as for the times I hurt her (I cheated several times, not sex but all the same )
I feel as though I owe her my love, almost.
And like before, I can't stand the idea of her being sad. I know she'd cling to the hope of getting me back.
Do I stay and how things get better? I feel like I'm being crushed by this issue. Please help.