I've been through this with my own parents, not to mention all four of my brothers have too. I'm now 28, married with a two year old. My brothers are 30, 25, 22, and 21. I've seen a lot of people who said it's ok to have a "my house, my rules" concept and I don't agree with this. There comes a point in time when you have to let your kids be adults.
If you want your son coming home at a decent hour, don't make it about YOUR rules. Make it about courtesy for others who live in the house. For example: "I'd appreciate it if you came in the house by 1am because it wakes me (or the father) when you come in. If you can't be home by that time, I'd appreciate if you spent the night elsewhere and called to let me know so I don't worry that the reason you're not home is because you've been in an accident."
Telling them they have a curfew because you're their parent and it's your house is pretty ridiculous if he doesn't live there full-time, or even half-time. If he's living on his own elsewhere, then you have to trust that he knows how to get himself home to bed without you telling him so why would you feel the need to "parent" him when he's home? Again, it shouldn't be about what you think is best for him, but about what you think is courteous to the others. HE has to decide on his own what is best for him. It's part of being an adult.
By saying "my house, my rules" you're telling them that it's not their home and that they're only welcome there when they're willing to do what you want. That might not be what you meant, but that's the way they take it. So, my advice, make it about being courteous of others. My youngest brother's gf lives in my parent's house, along with the bro, but they don't share rooms and they aren't allowed to sleep in the other's room. This is courtesy for the youngest siblings (age 12 and 13). It's a "bad influence".
Sometimes you just have to let go. Understand that your son is now an adult and needs to learn to solve his own problems. Your role is to be supportive and give a helping hand only when asked. Otherwise, he needs to stand on his own two feet. I wouldn't worry so much about the gf. Trust me on this, there are thousands of girls at his college that are going to have much more in common with him that the old gf doesn't. The other girls will have common professors, common studies, common friends, common school interests, common college-related problems, common goals. He's going to grow apart from her and want a gf who is closer and has more things in common with him.
Long-distant relations are difficult to maintain. She already complains about seeing him only 3 days a week. It's going to become less and less until one or both decide it's just not working and they want someone who is closer and more convenient to see. She's been checking his email and interfering with his relationship with others. That's a HUGE sign that she already knows he's growing apart from her and has a life separate from her. Too bad for her. She should have known from the minute he said he was going away for college that their relationship was doomed. Happens nearly every time.
I'm sorry you have trouble sleeping because you worry so much about your son. Sounds to me like you're a caring mother who loves her son very much and only wants what's best for him. Though I'm not the oldest of my siblings, I was the oldest in the house (older brother lived with out father). Our mother had to go through just what you are going through, minus the gf/bf. She held on too tightly and I felt I had to force her to let go so I went to college on the other side of the country and stayed away for a year. It took a very long time for her to adapt to her little girl being an adult who makes her own decisions. But once she adapted, she was fantastic. I tell everyone that I have the best mother in the world. She's raised 8 kids, four of us in/finished college, three are too young for college, and one has a very good job with a very wonderful wife with twins on the way. And none of had a curfew after turning 18. Well, except me, but I moved away so I could learn to be an adult without another adult telling me how to do it. :)
Good luck. Being a parent is the hardest job in the world, no matter what age your kids are. It's a full-time, permanent position. Just keep in mind, once they reach adult-hood, it's time to go part-time and let them start doing some of the work to raise themselves.