Wanted to add something... Marty you mentioned that Sam is too young to be serious. I'm not sure how old you are Sam, but this could be true, or not..
Everyone matures at their own pace...to put it concisely, I was through the wringer and out in EVERY facet of my life by the age of 18. I'm no guru of life, but I am more experienced in both the good and bad aspects of human existence than most people 5-7 years my senior. To be honest, I wish to GOD I weren't, but **** happens, right?
My point is, I'm 21, and engaged, and I'm ready. My female neighbor is in her mid twenties, and I have a feeling she is about a decade or so away from being ready to settle down
Some people wanna party down all the way through grad school, and others are ready for a more "quiet" life by their junior year in college. The only thing you need to make sure of is that YOU know youre ready, and that you've picked the right person. I can tell you have learned from this experience just by the fact that yeah, you took her back once, but now you've realized she IS NOT good gf material. That speaks LOADS about your ability to make mistakes and learn from them
Marty is wise in advising you not to rush it. You'll know when you're ready for an ultra-serious relationship, and you will also know when its time to "pop" that question, and if your girl is ready as well