It's natural to worry about getting a visa (no matter what kind!), but your case sounds like it's on the up-and-up. In my opinion, it'd be best for you to get a spousal visa (meaning your fiancee' would fly to the US, you'd get married and apply for your spousal visa in the US, and then you'd both return to the UK). I say the spousal visa rather than the fiance visa because financial issues seem to be important to you -- with a spousal visa you can work immediately, whereas with a fiance visa you'll be unemployed for up to six months (or longer!). If you're asked why you "skipped" the fiance visa (you won't be asked), you could just give that for a reason--you wanted to be able to work immediately to contribute to your shared expenses.
My husband and I went the spousal visa route and we weren't asked about our relationship history at all. We were asked for the basic documents (see my site below) and sent on our merry way. I think that there's less suspicion surrounding a spousal visa because you've already exhibited some level of committment by marrying the person.
Your age difference, while substantial, isn't one that's overly suspicious. If she were 70 and you were 30, you might run into trouble, but 15 years isn't too much of a difference when you really think about it.
Also, the fact that you've already lived together in the UK really strengthens your case. You have shown a desire to get settled and learn about the UK and your girlfriend has supported you (and/or you've relied on savings) for the duration of your stay.