Four years ago, after the last London Mayoral election, I was packing up the Camden Green Party pop-up shop when I received a phone call that changed my life - the position of Green Party Leader was open.
A month later, having been persuaded to stand by many party members, I was setting out my aims. Those included: becoming "a truly national party that leaves no region untouched", "getting in major national debates", "growing the party membership and supporting local parties" and building a "party fit for purpose".
Those aims, I'm proud to say, have been achieved over my two terms as leader.
Membership was about 13,000 when I stood to be leader; it's now more than quadrupled. With a vastly larger membership we have greatly expanded the staff team to provide more support to local parties, to members, for fundraising efforts, and to getting our message out.
We have scores of new parties across England and Wales that ensure most voters now have an active local Green Party. Supporting those local parties has been one of my major priorities and it's why I've been on an almost non-stop tour of the country, helping us grow into new areas.
We won 1.1million votes in the General Election last year, more than all the votes we'd won in every previous General Election put together. As well as retaining Caroline Lucas in Brighton Pavilion, we finished second in seats in Bristol, Sheffield, Manchester and Liverpool, something we'd never previously achieved. We saved our deposits in 123 seats compared to six seats in 2010.
In the first televised Leaders' Debate I looked David Cameron in the eye, challenging him directly about his policy on Syrian refugees - and then took on Ed Miliband and Nigel Farage when they tried to talk over me in the second debate. And yes, I did get to enjoy "that hug" with Nicola Sturgeon and Leanne Wood - providing a model of a different way of doing politics that I know has encouraged many around the country.
Standing up to the anti-immigration rhetoric is one of the areas the Green Party is now well-known for. More, there's increasing public understanding that when you want the honest, caring, committed view - one that isn't guided by the views of the latest focus group or fear of a tabloid backlash but by fundamental principles and values - you should come to the Green Party.
I was proud of my party when it produced "refugees welcome" mugs as others were ramping up the anti-immigration rhetoric. I was proud of my party as I presented its view on successive governments' treatment of benefits recipients, calling for decent benefits to be given gladly to all who need them, saying that the Work Capability Assessment should be abolished.
Now the Green Party gets consulted on the full range of policy areas, from welfare (and our policy of universal basic income is winning increasing support) to the NHS, housing to banking (with our new South West MEP Molly Scott Cato winning plaudits for her contributions in this field).
There's been a great deal of progress in the past four years but I feel it's now time for the Green Party to have a fresh debate about the direction we want to take, what we can achieve in a turbulent political climate that's full of opportunities and also carries a heavy responsibility - the responsibility to deliver on what I believe is the only political philosophy that can tackle the multiple crises that Britain and the world face.
I know some will find my decision not to re-stand for the leadership hard to understand, and I've been moved by the generous words of support from many party members and supporters urging me to continue, but I hope that my decision will help make it clear that the Green Party doesn't operate like other political parties, with a steep hierarchy up which many are seeking to scramble, while those at the top defend their positions. We're a team, we work together and support each other.
Of course my four years as leader haven't been all plain sailing and as the party began to be taken more seriously than ever the level of media scrutiny and, at times hostility, has been gruelling. I've always taken my responsibility to the party seriously and have done my best to live up to rapidly escalating scrutiny. I've learned much from both the times I got it right and the things I got wrong.
I'm not a smooth, spin-trained, lifetime politician. When I joined the Green Party on the first of January 2006, I wasn't lining myself up for a new career; I simply wanted to do something about the state of our world.
One phrase I've become known for is "politics should be something you do, not have done to you". For that to be widely implemented, we need to change our political culture and the media circus, away from the adversarial, aggressive point-scoring towards a more cooperative, honest, straightforward endeavour.
We also need to stop seeing politicians' families as fair game for media attacks - a particularly pernicious element in the British media that astonishes other parts of Europe.
Politics should not be a game for the privileged few where schoolyard bullying rules the day but must be a debate about our collective future that draws in voices from every area of society.
Changing our politics is going to need proportional representation, a fair electoral system that will force politicians to work together even in disagreement. And it's going to require media change - development of vibrant, effective alternatives that will leave the dinosaurs behind.
Not re-standing for leader doesn't mean that I'm going away, or stopping my full-time engagement with politics. I'm planning to keep travelling the country supporting local Green Parties and campaigners who ask me to do that. I've got policy areas that I'd like to get more involved in: education (influenced by the many young people I've met who are passionate and angry about the damage the system has done to them), food, farming and broader land management issues and the overarching topic of the transformation of our economy to one in which everyone can live without fear of want without destroying the planet.
I'll be remaining leader until my term finishes at the end of August, so I'll be going all out in the coming weeks on our campaign to remain in the EU, and continuing to support local parties and campaigns.
I want to finish with a "thank you". I've operated as Green Party leader with a tiny fraction of the staff support and resources of other party leaders. That's only been possible because thousands of Green Party members, supporters and non-party campaigners have helped out: arranged visits and logistics, briefed me and sent information, alerted me to opportunities, put me up in their spare rooms, and on occasions acted like professional media managers without any background experience. You've been brilliant, thank you.
It's been a privilege and an honour to have the title "leader of the Green Party", but every member of the Green Party is a leader, helping to lead the way towards a society in which we live within our environmental limits while ensuring no one fears hunger or want.