In my experience, most people do not say yes to opportunities because they lack fundamental self-esteem -- they don't think they deserve it. It's very common. Freelancers constantly underprice themselves -- same problem.
In this capitalistic society, we measure values of services and products by the price tag. That's why cheaper is not always the better deal, and that just about every business instruction tells you to price it fairly and deliver more value.
If you do not accept that you yourself, and what you do, have value, then you'll go on being a valueless (worthless) existence.
Tragically unnecessary, if you ask me.
But I myself used to be like that -- and it took me a good year or two to get that script out of my system. It was a simple rewriting of erroneous belief, but it was in me so strongly, the habit so deep, that it took a lot of conscious efforts. But it's all internal work -- it doesn't depend on any change to happen externally. Actually, as with everything else, outside changes follow the inner changes.
If a miserable sulker like me could change, so can anyone else!