Could you hold a conversation while you're running? If not, you're trying too hard.
Any training plan is a guide. Might be worth going back to the couch to 5k again but do each week twice if necessary. When you reach the point where it all goes pear shaped, repeat the previous week until it feels easy before moving on to the next week and do that one several times if necessary.
It may be that you need to take a step backwards in order to move forwards.
Oh, and the 'runners high' is an elusive beastie. Sometimes it happens, sometimes it doesn't