Sometimes I think it can be a habit. We can get into a habit of expecting things to go wrong and someone one said 'what I most feared, happened' and I think that if we expect things to go wrong then we're almost making them go wrong by anticipating them.
Our minds are very powerful. I mean we don't just get 'turned on' by someone we fancy who walks past us. We look at them and then we think thoughts and that increases our arousal. I think it's similar with other stuff too. I mean you can almost feel the energy of someone who is depressed, it's expressed in their facial expression very often anyway and by what they say and the mood around them, but it almost pushes people away from them too.
People who are happy and bubbly often have lots of friends because it's an attractive energy. I'm not saying that you're a negative person but I think that something negative has got a hold of your mind when it comes to relationships that's all.
I don't know if anything I'm saying is helpful but I know that in my life I've had to make a decision (many decisions) about various feelings and thoughts I've had and it's moved me forward and helped me be back in control of my fears.
Try and enjoy what you have now and today and try not to spoil it by worrying about what might happen tomorrow. Remember how important you are in G-d's eyes and focus on Him and not so much about your relationship. My confidence has come from Him and who He is rather than what He can do to help me. Then when I look at my life I can see how He has been helping me without me even being aware of it. Hope that's makes sense.