What are some good methods of gaining stamina?

I smoke and drank heavily for the past few years. One day I decided to quit both and get back into shape and live a healthy lifestyle. I have been jogging everyday.I also box at an amateur level. I am moderately strong and fast, I get spooked in matches and sometimes back off from fear of over exerting and losing my breathe, I have been lucky the past few matches, because my opponents made bad mistakes giving me an easy KO opportunity. I know for a fact that if I get in the ring with someone that knows what they are doing I will get my *** handed to me, my biggest issue is the stamina, If I can increase that I would be quite a bit better. It's not that I have no stamina, I just want to increase it. Any tips would really help. Thanks guys