Poor Self Image- I'd love to have to confidence

I'm a 27 year old man who is currently in the middle of one of his worst barren spells in terms of relationships.

I've had a couple of relationships in the past, and one that nearly went down the road of engagement. I have been out with a few people since this, but I have now found myself feeling completely withdrawn and unable to ask out girls.

I can talk to girls and mess around with them, many of my friends are girls, but when it comes to flirting and asking them out I just shell up and lose all confidence. It's getting to the stage now where I'm actually embarrassed to go out with my friends as they are all trying to set me up or get me back into the dating scene but my mind feels like it is completely poisoned.

I'd love to have to confidence some guys have to walk over to girls and ask them out, but every time I do i'm convinced that I'm fat, ugly, have nothing to offer, and they will reject me. The worst thing is that when I'm not in this scenario I don't think this at all!

Any help or advice would be greatly appreciated.