Been there done that. As a 26 year old I am a little removed from your age group but not much. Here is what I was thinking while reading.
First of all if you have feelings for someone, you should tell that person yourself. It may seem more difficult or it may be much easier to send a friend .to do so, but it is the right way to do things.
Second of all your 18. Crushes come and go. Everyone has been in a situation similar to where you have been. It may not be as "dramatic" as yours is but we all have been there. If you value the both of your friends friendship then it is worth continuing to talk with them. Who knows what will happen in the future. Maybe you will get together with the Alyssa, maybe not. If you enjoy talking to her but that is it you may find someone else better than Alyssa (Even though you think that isn't possible).
Finally you are in college enjoy yourself. It really is a completely different experience. It isn't all drugs, sex, booze in college. While I did enough boozing in college I still had time to pursue other avenues. Try going to the gym, try playing a rec sport, hang out with your doormates (I am assuming that you live in the dorms), join a club, etc. Find a hobby while up at school, something that you enjoy to take your life away from all the "drama".
As an old and wise 26 year old
: enjoy yourself in college.