I thought a little about Romantic Interest's reasons why our relationship could not work out, and I also thought a little about the boy last year. I came up with this:

If I may be so bold as to make a small suggestion to Romantic Interest, Boy Last Year, and all those who have found "the one" in their lives: QUIT DATING.
I mean it.
If you think that you're going to be with someone else for the longest time, if you think you can't live without that one particular person, or can't for reasons leave that person, or think you've met the magical someone whether or not you are physically together, you do not belong in the available circle. Good God, give it up. It ain't for you.
The playground rules are:
1) you are available
2) you are single
3) all of the above.
It's as simple as that. Don't come in pretending to be it, when you have "the one" in the back of your mind, "the girlfriend" you are (un)fortunately obligated to, or the love you never forgot. Just do not violate those rules because when you do, poor innocent parties get hurt. At this table, we play with an open hand, and a clean hand. We do not sneak cards in later in the game.
There is no such thing as somewhat single, a little single, a bit available, just like there is no such thing as being a little pregnant. You are either single or not.
It ain't that complicated. Just play by the damn rules.