Why can't I feel strong positive emotions anymore?

For a while, I've been noticing that nothing really excites me anymore. Two years ago, we went to England, and while I was somewhat excited, the worry factors overpowered it. What was going to go wrong? Was my husband going to have a good time? Now that I look back, the first things that come to my mind about the trip were the things that did go wrong. I think of the crying babies that kept us awake all night on the flight over. I remember that I didn't write down the address of one of the B&Bs, so I had to go find an internet café, and Tim decided to wait with the luggage in the underground, so every minute I was gone, I knew that he was worrying and waiting and wondering. Those memories overpower the good memories like seeing Kenneth Branagh in Ivanov and David Tennant in Hamlet, of visiting the Doctor Who exhibit and having lunch in an outdoor market, and walking along the Thames until we found the London Bridge.

Recently, I was in a play, and I arrived for opening night, and I was a little worried about myself because I didn't have that nervous anticipation, the excitement, the little butterflies that usually accompanied an opening night for me. When I arrived at the theatre and saw the buzz of activity, a few patrons waiting in the lobby, I started feeling a little excited, but not THE feelings I used to have when I performed in theatre years ago. This was the biggest role I've done in years. When I did Short Attention Span Theatre and my scene was maybe 8 minutes long, max, I thought I wasn't nervous at all because it was just a tiny part of the evening. This time, I was in lot of scenes and I still didn't have that energy that comes from opening night.

The only strong emotions I feel are the negative ones. When I worry or get stressed, then I start crying, then my chest tightens up. Although now that I'm writing this and thinking about it, those times even come farther apart than they used to.

Except that I don't want the expense, I should probably see a psychiatrist. I am so tired of never feeling good about myself. When something good comes along, I can't fully enjoy it. It's like most of my emotional nerve endings are dead, and the ones that are still alive only transmit negative energy.

What can I do to be happy?