With the ex situation it sounds like you might be just lonely - only because you say he wasn't very nice to you. But this time of year, after the holidays are all over, can definitely be hard and can make you think back and go "hmmmm, maybe that person wasn't so bad....at least there was SOMEONE there..." But only you know for sure whether it's really HIM you miss, or just being with someone.
Today was my first day back at school (university) after the Christmas break. I went to my Mum's for the holidays to get out of the city and "try to relax" but I ended up feeling very isolated too. If you live in a rural area and don't drive or don't have a car it makes you feel like you are stuck there. I do drive, luckily, but I still felt alone. However, I feel alone in the city, too, so that's nothing new.
I think the notebook is very good idea. It gives you something to do, a place to put down whatever you are feeling, and then you have a record of what was going on on a particular day, how you were feeling, what helped, what didn't. You might see some patterns coming out of it.
When I get anxious I find that distraction helps. Something that I really have to focus on, like rolling coins sometimes works for me. I have spent many hours rolling pennies as a way of staying occupied. If I'm feeling extremely anxious I can't really focus on words on a page, so reading doesn't work, but something like flipping through stations on the radio and seeing if I can identify the songs helps. I also find calling and talking to someone is good - it distracts me from what I'm thinking, and it lets me focus on something and someone else instead of myself.
Try not to worry about school right now and just relax. Is there anything you like to do at your parents that you haven't done lately? I like to take baths and read magazines when it's quiet and there's nothing else to do. Or even watch bad t.v.....I get sucked into those home renovating/redecoration ones. I have no idea why.