Losing mindfulness and peace after three weeks of successful practise I've been practising mindfulness meditation for about three weeks now. I started with ten minute sessions but soon increased it to a half hour and then either a full hour or two haf hours with a five minute break. I felt that my practise was making me more peaceful, less angry, calmer and generally happier. I would achieve what access concentration quite easily and yesterday I felt an experience that I found rather similar to the descriptions of the Buddhist first jhana.
Immediately after that experience, it seemed to me like my mind had almost, and very suddenly, let go of the progress that I had made and I suddenly returned to my former angry restless self. I am again suddenly consumed by angry thoughts about a trivial event that happened many months ago. I don't feel the peace that meditation used to bring anymore and today I really struggled even to finish a half hour sitting. I don't seem to be able to release distractions or thoughts anymore and can't stay with my object of meditation.
What's going on? What do I have to do to restore my mindfulness?
All of those things are coming back because you haven't really fixed anything.  So many people use meditation as a vacation.  They run themselves ragged in their day to day lives only to use the cushion as a kind of karmic hospital.  This is palliative practice and just like a vacation from your job or your terrible marriage, spending a week in the zendo or Tahiti isn't going to do a damn thing for your mind.  Meditating this way isn't any different from popping pills or smoking a blunt.  Sure, your problems melt away for a time, but you aren't addressing the crap that made you want to melt them away in the first place.
We sit to prepare our minds for insight.  Anything else that happens - jhana, bliss, IDK talking to devas - all that is wholly ancillary.  Rather than try to preserve those feeling of bliss, wholeness, and peace, instead turn that bright and less blemished mind to your own personal obstacles.  There are entire suites of insight, whole canons of material that you can investigate from Zen koans to vipassana objects, etc.  Gaining direct knowledge into those things are what's going to stop those bad habits from reemerging.  Simply sitting isn't going to cut it.  Bliss doesn't change anything.
And for what it's worth, you're just starting.  Of course you are going to hit a roadblock.  I've been doing this stuff for the better part of two decades and I still have good practice days and bad ones.  Keep at it.  It'll come back.