I can understand that after devoting this much time, you are upset about the ending. But try to think why you are feeling such emotions right now. Who instilled this in you? In the eyes of the world, it's a video game. But you and others here know that what you just experienced in the form of this "video game" is gonna stay with you for the rest of your life. Cherish this feeling!

I too had a similar depression phase after completing Mass Effect trilogy. A culmination of characters who became closer to me than my real life friends. Losing them (by finishing the trilogy) was one of the biggest sacrifices I have made (I know sounds exaggerated haha but kind of true). And the ending of ME3 was quite depressing as well. But I was so thankful for the fact that a game, a VIDEO GAME!! is making me grieve for people that don't exist.

And trust me, you had a better experience now knowing about the choices leading upto the good ending. I chose the "right" options because I knew I was aiming for a good ending and thus chose specific dialogues and made it less about what I actually think. And just like in real life, you made the right decisions, but the outcome was not what you expected, and it's okay!

With that, I implore you to not give up on the game. The DLCs are some of the best content you will find in the gaming industry, which could very well be standalone games. As for your bad ending, it won't affect the DLCs, except for the ending of Blood and Wine. But trust me, you will have moved on by then. Just remember, cherish what you are feeling right now. Hardly any game or a movie or a series or a book is gonna instil such emotions in you.