It sounds like your personal anxiety is creating relationship problems. The anxiety seems to be coming from what you describe as 1) “nagging doubts” about the relationship, as well as 2) fear of the relationship ending. People can have doubts and enjoy a healthy, long-term relationship. Doubts and happiness don’t have to be mutually exclusive. We live in a world full of uncertainty so having doubts isn’t a bad thing — it’s normal. However… your behavior regarding these doubts is what will affect the relationship. So, if you can have doubts and be okay in the relationship, then all will be fine. But if your doubts become panic, these panic attacks are more effective at gnawing away at a healthy relationship. That’s why you have to figure out what the panic is about and whether or not you can calm yourself down. You didn’t mention what the doubts about the relationship are, so I can’t help you there, so much. But you did say you were afraid of the relationship ending, and that’s understandable, but when you realize that relationships sometimes do end, and break ups and rejection hurt, and that by being in a relationship these discomforts are always a possibility, you’ll be closer to finding peace with reality. Nobody likes a break up or rejection, but people get over them and go on to find love again, and once you realize that you’re strong enough to be in a relationship that may or may not end, you’re going to be able to let go of your panic.
I hope that helps!