Why am I letting it happen?

Read your own post. What do you see?

I see that you’ve already made up your mind that it is going to all go downhill eventually. What you think, is what you’ll draw to yourself and the situation. You must not go into a relationship with a negative attitude. It will fail. Try changing how you approach things. Think positive.

Because that’s how you learn ........................

And I hear those are memories to look back on in joyful memories of good times.... and dreadful mishaps.

what else is there in life that can make you feel so good?

Because you're a human and we're all nuggin futs.