I struggle with this, too. My anxiety looking forward to good things is almost always equal in severity and discomfort to that I experience when managing something reasonably stressful for anyone. Fortunately, I've spent a good amount of time wondering how I can curve this issue which is why I'm happy to comment on your post.

The thing I've found that works most of all is to keep busy with other things and not allow myself to attempt to live out anxieties in my mind. Practice distraction and shower my confidence in healthy affirmations should my mind refuse to budge. Start or work on projects that are independent from what I am anticipating. Basically just do your best to switch focus. You can try and analyze and dissect why you feel the way you do but it usually just comes back to the fact that you are suffering which in turn rightfully validates self pity and sadness, or at least that's how it seems to me. The most proactive thing I can do at that point is to remember there are other things going on in my life, many of which are fun and enjoyable.

If you anxieties become obsessive, don't hesitate to dive into comforting affirmations. Whether or not you believe that you will enjoy what is about to happen, your mind will take notice to the fact that you are at least trying to see things from another perspective.

I am sorry to hear your parents fought so much and that you went through traumatic instability. It is very difficult to make sense of such deep rooted pain and emotional neglect. Regardless, your parents actions and the trauma you have endured does not ultimately define who you are as an adult. You have everything within you that it takes to make it through this challenge far stronger and happier than you ever imagined and I am hopeful you can see that soon.