Lack of confidence & Mistrust

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 When I was younger I was fairly outgoing and social, and as I have got older I have become more and more withdrawn to the point where I have no confidence in myself at all.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.

Maybe this is really how YOU feel about HIM? Are you hoping to find something better and just keeping company with him until someone better comes along? I am from the previous generation, and this was NOT how we envisioned things for you guys! We wanted you to feel free to live with someone you LOVED, not be tied down to someone you weren't sure about. That seems about as bad as being unhappily married. This was not why we burned our bras. Either grab him and know and be happy you have him, or let him go. We believed the piece of paper was irrelevant. It doesn't change what the truth is.