What makes you unsure he won't be faithful? Has he given you any reason, in the 3 years you've been together, to doubt him? If not then I don't think it would be very fair to end the relationship on a doubt which may be unfounded.
I've just started a new relationship which is already long distance and is about to get even longer and I've never had a long distance relationship before so I've got fears and doubts, but I think he's worth it so I'm willing to try and see how we go.
However, university is one of the best times of your life for new experiences and meeting new people so you have to take that into account as well.
I'd give it a try if I were you, the most important thing you can do is keep talking to each other! To be honest sounds to me like you've already made your mind up!