Nice guy or not, just be yourself. If got rejected, don't blame the girl or anyone, instead blame on oneself for being stupid on falling for the wrong girl or maybe something you did wrong along the way.

Concerntrate on your career and studies like the above mentioned, your network and your outlook. Always think of improving without neglecting your good point. Friendzone or doormat or whatever, just do what you know is correct. Nice guy is never a sin instead he is the person to help people around him. Many girls may find you boring or dull but in times of trouble, you did help them out. What i can say is, at the end, you have no regret in life because you have tried your best.