I didn't look at your location, didn't realize you were in China!! Having never heard of the brand of bike you're riding on it "seems" like the frame is stout enough, (note, I said seems). Having never shopped in China for bikes I'm not sure how to advise you because I don't know what the offerings are over there. Like you said $50 is a lot of money in China, so you could probably get a really nice bike for $200 there. Problem is due to the cost of things there wages are usually proportionate with wages, thus $200 may be difficult to obtain for a lot of people there, but I also don't know your income level either.

Sorry, I just can't advise now, other then try looking for a bike you think is higher quality for a price you can afford. I do know this, you can't keep having breakdowns, even though you've been lucky up to now the breakdowns have been on the way home, next time it may not.