The antagonist is the person (or thing) in the way of your protag getting what they want. So I'd suggest looking at what your protag wants first. That way, you'll have a better idea of what to out in their way and why.
The book is about your protag getting that thing, whatever it may be. What are things that could be obstacles to them doing that?
Is there a person that doesn't want them to get it? (a man vs man setup)
Is there a personal hangup that preventing them from this? (a man vs self setup)
Is nature turning against them? (a man vs nature setup)
I suspect that you don't know exactly what it is your protag is trying to accomplish, because accomplishing X would be the ending of your book, and you say you're having trouble coming up with a good plot ending. You probably have a general idea of what they want, but nothing specific that's driving your protag to act, so you can't find what in their way of acting. For example, "they want to find happiness" is a general idea, but that can be any number of things. "They want to reconcile with their estranged father" is a specific goal.
What is your book about? The one sentence (doesn't have to be a good sentence) that describes the core conflict of your book? Your ending will be when your protag resolves that conflict.
Now, who or what is in their way of getting that? The big ultimate "in the way," not just smaller problems that come up in the story. The one person or thing they'll have to overcome or beat to win what they want. That'll be your antag.
Once you find that, then pretend for a bit that your antag is your main character. Think about what they're trying to do and why, and what things they might do to get that, same as if it were your protag. Flip it so your protag becomes you r antag for this exercise. This should give you ideas on antag motivations, what they might do to stop your protag, and ways to plot.
If you only focus in what your antag can do to your protag, you often end up with flat, two dimensional antags, because they don't have goals or motives in their own right. But if you approach them like a character with legitimate wants, they're much easier to flesh out.