Hi Sheenah,
It sounds like you have it very tough. I can see a number of problems here. Some of them have been mentioned, but a few haven’t.
As far as school is concerned, I think your kid needs special attention. Most current school systems deal badly with people who fall too far out of the average. This goes for children who have trouble keeping up because they lack the mental skill or experience (lack of parental support), but this also goes for children who are too far above average. They are often bored and left to their own devices to do something about that. As a result, they can become bullies, trouble-kids who continually challenge authority, and so on. From what I hear the teachers say about him, he could fit that bill. To deal with his, you need to find something extra to challenge him with. You might want to try to find some bright-kid programs in your area, and/or open a separate thread here asking if there are people here who have experience. This is not necessarily something for ‘gifted’ kids, just something that offers an extra challenge.
At the same time, your child has started the final phase of becoming an adult. This does not only mean hormone levels start to rise (which is a problem for a while, it reduces your attention span to 20 minutes max, and with classes typically at least double that, it causes problems for a year or sometimes two), but it also means that mentally, the child is working hard to become independent. Since your son is currently obviously the most dependent on you, you’re going to have to bear the brunt of the burden in that respect.
I think that all things considered, if it were an option for the boy to go live with his father, I would hold it open for him. Don’t allow it to be blackmail material. Just be honest in telling him how you think about it, what you are afraid of for him (what happens if his dad rejects him, finally, can he deal with it? Asking this question will force him to deal with it a little beforehand, which will in itself make it a slight bit easier to handle), and for yourself (can you handle him not being around). But start moving towards treating him as a grown up that has to make his own decisions and bear his own responsibility for them. Typically, if you do this before they force the responsibility from you, my experience is that they’ll be more careful and likely to listen to you for advice than otherwise.
I imagine the most difficult part for you to deal with would be if he does in fact move to his father, and he likes it there and doesn’t come back. You would lose his company, terrible, but something that will happen sooner or later anyway - he will move out. If he in fact is happy with his father, you may feel that you have failed, but at the same time your son is happy, and even if you’re not, that’s not a bad thing in itself (“I would terribly miss you, but if you think you would be happier there, you being happy is the most important thing to me in the world even if I would be unhappy not having you around” - if you mean this, your son will hear you, even if he still decides to go).
And even in that case, which you must find very threatening at the moment, it would allow you to deal with that other important thing you mentioned, the fact that you feel you’ve been depressed the last three years. That’s not a good thing. Whether or not your son stays, you should really prioritize doing something about that. It is hard to love someone who is depressed, as I’m sure a large part of the SD population is able to tell you. Get those things that made you happy in the past back out of the closet, or find new things.
It must be hard to do these things on your own, though it sounds like you have one or two friends in similar positions. You may want to go out there and look for more of them, or make some new friends just to have some fun with or give each other support. I hear the internet’s a good thing sometimes.