Depressed Local Runner- I'm very sorry to hear about your situation. Anyone who says it's easy to get over a breakup/divorce hasn't been through one, at least not one as big as yours.
After reading your posts in this thread, I have a few suggestions.
First, cut off contact. Defriend her on facebook. Unfollow her on twitter. Whatever contact you could possibly have with her, completely remove it. Don't write any more letters. For whatever reason, we guys think that the best way to get a woman back after a breakup is to drown them with loving messages, gifts, attention, etc. In fact, you want to do the opposite. You need to show her that you don't give a sh*t. Well, at first you're just making it seem like you don't give a sh*t. But that's what you have to do. Trust me on this one. Do not send any more letters or anything like that. Look at your calendar and circle the date exactly one month from now. You will commit to having ZERO contact with her between now and that date. It will be hard as hell, but you must do it.
Accept what happened, and accept that she probably is dating someone else. Your breakup is in the past. The only thing you can do is stay in the present and alter your future. I know you're still going to wonder what happened and what you could have done differently. We all do. But you have to be forward-thinking. In addition, your ex probably is dating someone else. Women don't typically leave their spouse/boyfriend until they have something else to go to. I know this is a really, really tough pill to swallow, but drill it in again: she is most likely dating someone else. However, this can give you the proper motivation to better yourself. This is the thing that brings guys to their knees- not necessarily that your ex left you, but that she left you for someone else. It hurts more than anything. But accepting it will make you a better man in the end, I promise you.
Better yourself. This applies to many things. Use all of this as motivation. Someone before suggested going to the gym and working out- this is great advice. You seem in to your running, but a little change of pace might be a good thing. Go improve your body. Get stronger muscularly. This doesn't mean become a meathead that can't run a mile under 7 minutes. Just get some definition going. You have to "fill in the gaps," one might say. Perhaps you became less social throughout your relationship. Maybe you let some friendships slide. Maybe you gave up on an instrument you were wanting to learn. Fix all of these things, and start new hobbies. Make yourself a better, more well-rounded man.
Go out, and though you might not be ready yet, f*ck other girls. One of the worst things you can do is sit around in your house by yourself. Get out there with your buddies and hit the bars/clubs/etc. I know you're in to your running and all, but f*ck recovery or whatever might be stopping you right now. You need to get your head straight and have some fun.
Now to the second part- getting with other girls. I don't really care if you date them or just sleep with them, you need to have interactions with the opposite sex. The PUA community has one piece of advice for anyone looking to get over (or get back) an ex: Sleep with twelve other women. That's all they say. Sleep with twelve other women. This is because right now you're experiencing what they refer to as "one-itis." You said it yourself- you're "idealizing" your ex. By sleeping with (or, again, just dating) other women, you'll gradually realize that your ex isn't all you've cracked her up to be. I know that sounds insane to you right now, but it's absolutely true. I'm not saying you'll think she sucks, but you won't put her up on a pedestal. And this isn't just important in getting over her- this is the only way to ever have a chance of getting her back.
When you first try to talk to girls, you will crash and burn. You must be okay with this. You've been out of the game for a long time and some broad just ripped your confidence out with her bare hands. This is completely okay. With time, practice, and patience, you'll get better at it. Also keep in mind that this is a positive outcome of your divorce- you can get with as many girls as you want! I know you only want your ex right now, but don't tell me for a second that you didn't fantasize about other girls while you were with your ex. Now you can actually do that, and it will be good for you. It will also make you gradually care less about her being with someone else.
Lastly, work on becoming a man. In all likelihood, your ex gradually took control of you to the point where you became her lapdog. This usually happens without you even realizing it, so you'll probably deny that statement. But that's typically what happens. Women say they want a guy who will be at their beck and call whenever they want. A guy who will give them massages whenever they ask, do all of the chores, constantly give them flowers and take them to nice dinners. A guy who wouldn't even glance at that hot waitress. A guy who would never even consider leaving them for someone else. This is complete, utter BS. This is what women SAY they want. It's not what they truly want. They want a man who is so confident in himself that he doesn't need a girl at all. And when he does have a girlfriend/wife, he feels completely comfortable knowing that he could drop her in a second for the barrage of other women who want him (ideally he loves her and does not, but that's beside the point). A guy that stands up for himself and doesn't always do what she wants. A guy who leads her rather than becoming her follower. They want a MAN.
So take on that mindset- you are a desirable man who was left by one woman because you got a bit lazy. That's really not a big deal. In fact, you probably needed this to get you back on the right track.
Use this to motivate you in all areas of life. You will be a better man in the future IF you follow this advice and keep a positive outlook as best you can. This is going to be extremely tough, as I'm sure it already has been.
Good luck. I'd like to help out as much as possible.