Hi @Corinna that sounds really disappointing. It can be really exhausting when you're stuck in cycles of relationships that don't work for you, and you're left questioning your choices. But the positive thing here is that you are recognising that there is a pattern, which means that there is going to be a way for you to figure this out.
It's great first up that you took that year out to focus on yourself without being in a relationship - how did you find that?
As for your main question, is there a way for you to think back over your relationships, with the benefit of hindsight, and think about that idealisation phase - was there anything you learned or experienced later that was there at the start, that perhaps you pushed aside?
It sounds like you know what idealisation feels like - is there a way for you to recognise you're in that phase as it is just starting to emerge when you meet a new person? Could you take a step back at that point?
A lot of questions in there, but I think you will be able to work through this in time, as hard and frustrating as it seems right now. It could be a mixture of the people you're attracting and your expectations, depending on the situation. If you wanted to talk through an example in this thread and get advice from others in the community that might be a good place to start if you're comfortable.