First off, if westerners abandoned their western lifestyles, humanity would be sunk: next to the collapse of aggregate demand that would ensue, our present economic problems would look very mild.

Universalizability arguments like this are non-utilitarian; it's the marginal utility of your decision (modulo Newcomblike situations) that matters.

The rub is trying to avoid being a part of really harmful, unsustainable things like commercial ocean fishing or low fuel-efficiency cars

It definitely seems to me that refraining from these things is so much less valuable than making substantial effective charitable contributions (preferably to existential risk reduction, of course, but still true of e.g. the best aid organizations), probably avoiding factory-farmed meat, and probably other things as well.