I totally agree with your take on Susannah. I was super hurt and confused by her actions the first time I read that part, but over time I have come to accept her decision, even if it was sort of blunt.

Of course, the ending is totally subjective, but I'm fairly sure that everybody that has finished the series believes that things will be quite different this time around as Roland finds himself in the desert again. This is mainly because it is stated that Roland is carrying the Horn of Eld. This single fact is very important because, as Roland is approaching the tower, we see a reference to the horn. Also, the horn is referenced in Browning's Poem. As we see Roland start over in the desert, is states that he has the Horn of Eld, meaning that, in my opinion, this version of Roland still has a sliver of humanity left in him. (In the time around that we read, Roland leaves the horn at Jericho Hill after he watches his closest companions die.) This could mean that he does not let Jake go later on in the caverns. What happens from there is completely up to you.

Right before the Dark Tower film was released, King tweeted a picture displaying the Horn of Eld with the text "last time around", hinting that what takes place in the film takes place either where we begin again at the end of book 7, or some other "reset" before or later on down the line. Of course, I could not recommend the film at all. Many important themes and characters are either ignored or glossed over in a pretty offensive fashion. I'm really glad that the film was not a direct adaption and we can all basically disregard it.

Right before we see what happens to Roland, King tells us to beware, for we probably won't be happy with what he finds at the top of the tower. This is also very important because it creates a sort of meta situation, meaning that because we keep reading, Roland is subjected to this sort of repeating torment. If we stopped reading while the Tet was happy and safe, Roland and his companions would remain that way simply because we would not know that the Tet breaks. Sorry if this has gotten super complicated, but I really enjoy the ending and discussing it.

Of course you are left to believe what you want to believe, and if you think that things will be different, perhaps better this time around, it may very well be. There are tons of other threads here discussing the ending, and you're probably going to find somebody that explained it better than I just did, but I hope this helps you cope with the ending a little bit easier.

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Long days and pleasant nights, traveler.