when i leave for college_becomes very depressed

I'm in the same boat.  My family has 2 dogs - one's the family dog and one's my dog.  I'm the one in the family who "does the dog stuff".  I trim the nails, give the exercise, brush their coats, etc etc, and when I leave...my family just doesn't really take as much responsibility as they should.  Cairo almost always ends up a little overweight over the winter, mainly because it's hard to take him for long walks and he doesn't like that many forms of exercise.  He won't run, he's not a large fan of fetch, and prefers mostly to just laze around.  In the summer I take him for at least one 30-40 min walk every day and he runs around and plays in the backyard with our other dog, but last year was my first year at college, so there's a whole chunk of the year right there where he doesn't get much exercise.  Whenever I come home from school (luckily it was frequently last year), the dogs would have very long nails and be in desperate need of a bath.  I haven't figured out quite yet what I'm going to to this coming semester, but I think I'll have a talk with my parents.  I aim to get an appartment soon at school and hope to bring Cairo with me maybe next year.

When i left for college I had many pets I had to leave behind, 2 dogs, a cat, snake, trancula, axolotl and my ducks (I'm shure I'm forgetting someone). I was the main care giver so I was a little worried my parents would not care for them like I did. Well, they did alright and my dogs did great but my cat would be missing for days at a time and she was already 15 years old. About 3 years into college she just never came home for a very long time and I knew that was it for her. I felt terrible that I could not be there her last few years of her life. I don't really have advice but I really do hope you work something out, it's almost to sad to think that you can't be there for them. I wish I would have taken my cat with me and really regret not trying harder. Maybe if you know a kid in the area that can kinda babysit your dog a few times a week then it could work.

first off, you might try a new 'scoop' or whatever you measure the food with and decrease the amount the dog will get while you're gone.  If the dog is going to have less exercise that only makes sense. 
 
You might try one year, seeing if you can rent an apartment so you can take him with you -- he still won't get the exercise because you'll be busy but it might eventually be an option.
 
He will get thru this period -- but be sure and have a talk with him before you leave and tell him when you WILL be home "I'll be home for Turkey Day - you know when everybody goes to ________'s house and we have BUNCHES of food?" or whatever days you celebrate.  No they don't understand ALL the word, but they do understand the concept and you'll be amazed at the difference.  See often when you leave that way they think you're not coming back UNLESS you teach them differently.
 
You might also see if your family actually lets him out at the sensible times -- like AFTER eating, sleeping, etc. -- they may not even think about the fact that if they let him out in the morning one time, he might have to go AGAIN after he eats.  Remind them that THEY always 'go' after eating or before leaving the house.  They may be leaving it to each other with the other assuming the first one did it. Then the dog gets the blame for 'accidents'.  Why not at least confine him to one room, like the kitchen if they're gonna be lazy.
 
You won't stop them FROM being lazy -- "it's your dog" and they think they're doing you a favor - and they are.  But just try to live with it and plan for the day you graduate and can get your own place.  But if you teach your dog 'holidays' -- and give them a time when you WILL be back, it will truly teach them more communication skills.
 
One time I had to go away for two whole months (not to school but a necessary trip) and I had my neighbor come over and let my dogs in and out and feed them and exercise them a bit.  We left on May 11 and were scheduled to be home like July 8 and I told my dogs "Mom'll be home AFTER Fireworks Day -- the day all the kids set off firecrackers BANG BANG BANG!"
 
My neighbor heard this exchange and teased me about how much I thot my dogs 'knew' and how silly it was.  til the day I came home from the trip.  She met me and said "I need to apologize to you"
 
Oh really?
 
"Yeah -- you know they never even LOOKED for you -- we came and went and it was NO big deal to them you were gone.  I was amazed.  UNTIL July 5th!!!  They did FINE over the 4th even tho there was a ton of fireworks on the street ... but the next day?? THEY WERE WAITING ... literally sitting at the door not wanting to even go out.  They did that every time we came on the 5th, 6th and 7th.  I did tell them 'Mom's coming home today' and they actually laid down IN FRONT OF THE DOOR!! Darnedest thing I've ever seen -- how DID you teach them that??"
 
Repetition.  When I'm gonna be gone for just a few days I show them 'fingers' of how many days I'll be gone.  No they don't understand math, but they have a concept of time passing and if they know it's a finite amount of time they worry less.  If you don't get home for Thanksgiving but do at Christmas tell them in relation to when the tree goes up if you celebrate that, or when the Menorah comes out if you are Jewish or whatever.  If your family waits for you to put up the tree that's perfect "When Dad gets out the Christmas boxes I'll be home!!"
 
But giving him a frame of reference will help a great deal.  Good luck.