If that is all you have heard all of your life, then you clearly associate with the wrong people.
I am on 'the left' and I haven't heard anyone who I associate with running this country down at all - in fact my only experience of people talking about how terrible this country is has come from people with a similar attitude to yourself.
What runs this country down is all of the people who talk about how terrible it is because we have such high immigration and hand benefits out like sweets - neither of which is provably true - we actually have one of the lowest paying benefit systems in Europe for both citizens and asylum seekers - and numbers of immigrants and asylum seekers only started going up after that idea started going public and Brexit happened. There is a direct correlation between Farage and pals shouting about how much asylum seekers get in the UK and their campaign for Brexit and the increase in the number of asylum seekers - they started shouting about a problem that didn't exist and turned it into a self-fulfilling prophecy!
I'm not sure what you mean by you have had a new identity 'imposed' upon you? Have you been forced to change your name and vocation? Has someone forced you to identify as an ethnicity or sexuality different to what you are? Has someone forced you to change gender or support left leaning politics when you are a conservative?
This country has an amazing culture - for a small island, our contribution to internationally successful literature, film, music, theatre etc is astonishing - JK Rowling, JRR Tolkein, Oasis, Beatles, Rolling Stones, Sir Andrew Lloyd Webber etc. Our sense of humour is fabulous and up until around 2014 our sense of fairness and inclusivity was something to be proud of.
We have amazing national heroes across culture and history - from Shakespeare and Jane Austin, to war time heroes like Churchill (obviously) and Sir Nicholas Wilson and Alan Turing.
We were (and still are in many areas) world leading with people like Brunnel and Stephenson being responsible for changing the world through their genius and innovation. You wouldn't be spilling your absolute tripe on Mumsnet without Ada Lovelace, Charles Babbage, Sir Tim Berniers Lee or Donald Davis
While it clearly has its problems now our establishment of the NHS and welfare state providing a cradle to grave social safety net was pioneering and if you have ever had to have anti-biotics, just bear in mind that you would likely be dead without Alexander Flemming. If you know anyone that has had a baby through IVF, they have Anne Mclaren to thank for that!
Everyone on the 'left' that I know fully understands and appreciates all of that.
They also understand the more problematic things that this country did in the past - recognising some of the atrocities that were committed in the name of 'empire', the pillaging and destruction of other countries wealth and cultures, the establishment of the Atlantic slave trade (even though we eventually led in ending it), the 'shipping' of criminals to Australia and the colonies.
It is still possible to recognise Churchill as a great war leader, whilst also acknowledging his extreme racism, advocation for the use of chemical weapons and dubious actions in Ireland - recognising those things doesn't take away his achievements - but it is a way of making clear to today's and tomorrow's leaders what we find acceptable and unacceptable in our leaders.
Understanding and recognising those problematic things does not mean that people hate their country - if anything, it means that they love their country more because they want to develop an understanding of the not so pleasant side to our history so we can make sure that it is not repeated, thus making us even better.
The only reason people would object to recognising and understanding those things is if you agree with them - and given, as a nation, we have generally been leaders, in more modern times at least, in promoting and pushing for human rights, then agreeing with those things makes you fundamentally less British, not more.