Hi, I am new to this!!

I know that I have a problem. When I was younger I was fairly outgoing and social, and as I have got older I have become more and more withdrawn to the point where I have no confidence in myself at all. I can't remember any trigger to this, I have never really been hurt in a relationship, and had a normal childhood with supportive parents.
I have always been insecure in relationships, but in my current one, it is causing real problems, but I can't stop it.
I question him all of the time, even though we have been together over 2 years, and check up on his phone, internet history etc etc.
I can never relax, and am always expecting him to hurt me, even though he has never given me any reason to think he would.
I have a sane part of my mind and an insane part, and when I get a paranoid thought in my head, I try and listen to the sane part. But then I always give in and need to question or accuse, so that he knows that I realise he is "up to something" which angers him. Once I have reflected, I realise that what I say has no basis or truth, but I can't take it back. But I can also not stop doing this.
The thing I am scared of is losing him, but the way I am acting is making that more likely.
I have been in relationships since I was 18 years old, and I am now 34, and I think I have forgotten who I am, but I don't know how to find that person again.
I am a plain Jane, but I know I am not "ugly" in reality, but I always expect my partner to find something better, or that he is only with me for company until something better comes along.
I am trying to take a good hard look at myself, do some soul searching, and find a cure, or a way to calm myself and stop the way that I act long enough for the sane part of me to resurface and take over, but in everything I read I can't find anything practical.
Please help me because I am making all my worst fears come true through my actions.