You may get more exact answers with a more exact question. What is your idea of "light"? For some it's 15oz, for others it's much more. How much money does it take to break the bank? $70? $200? There is a lot of leeway in your question.

You left out one very important requirement: How warm does it have to be? What kind of temps will it be used in?

Are you looking for down or synthetic? IMO down is required to make a bag truly ultralight but others may think differently.

As for durability, it generally rises with weight. Personally I think any bag can be durable with proper care.

As I have written about sleeping bags a few times before:

Light

Cheap

Warm

Pick any two. It's not hard to find a cheap warm bag but it won't be light. Sounds like you have that now. If you want a light warm bag it won't be cheap. And if you want cheap and light it won't be very warm.
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.