I'm sorry to hear about your recent breakup. I’m not entirely sure from your letter whether you are happy with your new status, or if you would actually rather be back with your former partner. As you say it was positive, I’ll assume that you are not looking to change the situation and are instead simply looking to change the way you are thinking and feeling about the situation.
It’s also not clear whether you are thinking about your partner, or experiencing thoughts of him. Those may sound like the same thing, but they're actually quite different. The former is where you still have a vested interest in thinking about him. There is a conscious effort to think things through. Maybe it’s wondering how he’s feeling, where he is, what went wrong, why you broke up, etc. It’s all very normal stuff, but maybe not all that helpful or productive in terms of moving on. The latter is where you are moving on with things, but every now and then the thought of him will arise in the mind. There is no conscious effort to think of him, but he pops up, nonetheless. As he has been such a big part of your life, it is no surprise that this would happen.
I’m guessing it is probably the latter and the negativity is arising because you want to move on and yet thoughts keep arising from the past. It’s as if the thoughts keep taking you back to how you were feeling, invariably tinged with sadness or nostalgia. I wonder what would happen if instead of resisting the thought and trying to move on, you sat with the feeling, allowing it to remain as long as it wants to?
Very often in life, we deal with difficulty by trying to get away from it. Even if the circumstances were positive, the end of a relationship is always sad and rarely clean and tidy. There are nearly always loose ends, unanswered questions, misgivings, and doubts. In our enthusiasm or desperation to move on, it can be tempting to neglect or resent thoughts and feelings which remind us of that time. And yet the mind needs that release. If we do not give it that release, we cannot let go.
So, my recommendation would be to give the mind all the space and time it needs to move on. It's as if you are providing a framework for the mind in which it can think all it wants without getting caught up in it. So it is not that you are thinking about your ex, but simply watching the process of release, of letting go. And don’t be at all surprised if that is painful and sad at times, even if you don’t feel it right now, as that can often be part of the process.
Hope that's helpful in some way and warm wishes, Andy