Okay, it's time for both a big hug and a slap round the chops with a wet kipper! 
I’m 42, I’m morbidly obese. I don’t have scales but I’m about a size 16 (which I know is a “real” size 20) and at 5’9 I just look like a giant. I must be about 15/16 stone.
If you are a size 16 and 5'9" you are NOT morbidly obese.
I think I may have developed lipedema too as my legs have become alarmingly enormous.
Get yourself to the doctor and get them to check.
I know that there’s no escaping that society sees obese people as the scum of the earth,
How can I be happy with my body when I know everyone else hates me for it?
Utter tosh, no one hates you or thinks that obese people are the scum of the earth, and the few that are judgmental are morons who don't matter one tiny bit and are probably the same people who think puppies are ugly!
I really am ok mental health wise at the moment. I do worry about relapsing and then of course going back into meds that cause me to grow even larger.
You really aren't if you are having these thoughts, why did you come off Sertraline, did you do it on the doctor's recommendations or was it your own decision? Not all antidepressants have the same weight-gain effects, I was on Citalopram and didn't have any problems and I have hypothyroidism so am prone to easy weight gain.
I just want to know what it’s like to look in the mirror and feel amazing! It must be such an incredible feeling?
Very few people look in the mirror and think they look amazing, even the prettiest people I know are critical of themselves!
I can’t really afford to pay for any private treatment with counsellors or therapists!
Get thee back to the doctor's and get on the NHS list for treatment.
I’ve tried my absolute hardest during lockdown to lose a bit of weight and I have lost some but it’s pointless really, as as even when I have the slightest slip up the weight literally piles back on so quickly.
Okay then, well done for starting but stop with the defeatist attitude, you are your own worst saboteur! Carry on with what you are doing. Baby steps. One less chunk of cheese, one less square of chocolate. One more walk to the corner of the street and back again, one more amble round the park. Gather the momentum and go further, stop trying to put obstacles in your own way, life will throw you the odd curve ball as it is without you helping! Sign up to MyFitnessPal (it's free) log your stats and follow what they recommend. If you don't have kitchen scales, invest in a digital set, cheap as chips on amazon! The discussion boards are great, go and have a read, you'll read something on there from people in the same and much worse situations. You'll find inspiration / hints and the odd kick up the behind from reading the posts. And if you have a question, ask! Lots of people on there are really knowledgeable and have had real life experience.
And after all that, remember - you are not worthless, you should not be ashamed BUT you are responsible for the decisions you make with regards to what you eat, how much you exercise and the mindset you have about your reflection! So..... decision time, I think you're worth a shot, do you?! All the very, very best to you! 