I don't think you failed the interview per se. Places for the program are probably limited and therefore the process is likely to be highly competitive. Is it possible to find out information about your performance in the final interview? Is there a contact at SQ that you dealt with who you can ask? If you find out what aspects of the assessment you didn't perform as well as others in, you may be able to address these in the future (depending on what your perceived/real shortcomings were assessed to be).
Can you use that university degree to get a job and complete training part-time? Lots of people want to fly but can't fund it immediately - if you want it enough, you can make it happen, cadet or no cadet program.
There won't be any certainties whichever path you go down, so think about the pros and cons of each approach and think about what will give you the best chance of getting to where you want to go. And IMHO, will make you happy even if you never make it to the airlines. Aviation is full of opinions, so listen to them, but don't forget about your most important one, your own, as the choices you make have to be right for you.
Good luck