Im trying to act normal but end up acting weird in front of colleagues

I definitely get where you're coming from & I can empathise, trust me you are not alone in what you're feeling. Lots of people feel out of place in all sorts of situations and social settings. What people think is 'weird' is very subjective & everyone has their own opinion on that. If you're good at what you do often what happens is everyone else becomes jealous & will try to make you feel emotionally uncomfortable. I think you should absolutely take pride in whatever you do, you harm noone & try your best. They obviously seem to have problems because why you would ever want to make a cooworker feel uncomfortable is beyond me, unless you're jealous or seriously lack social skills and empathy. Usually, in social situations one or two people have an opinion & spread that to others. Say if one of your colleagues were to tell the others they thought you were 'odd', the others would automatically be looking at you with a magnifying glass & try to 'spot' weird habits, but it's just not weird at all & you're totally normal. It's definitely not weird at all to post late & I hope you had a great birthday! Personally, I'd say focus on how brilliant you can be or are at work. Embrace yourself (easier said than done), try to up that confidence, cause no matter what you do or how normal you try to be deliberately, it's just never going to feel right and the others will always have an opinion. Purposefully smile even when you don't mean it, laugh out loud, listen to music/watch videos you love, your weirdness is unique as heck & the rest of them sound very boring to me. There's always a silver lining to everything, your colleagues clearly do not know the basics which are required when going into employment: have respect for everyone; work well as a team, not 'I'm going to subtly make this person feel bad'. Tell yourself, in your head, even if you don't believe it, that you're a boss & they can all shove their nasty attitudes in the bin. You have people in the NHS that don't know how to empathise with patients or make them feel better, same applies with people working in mental health. They literally don't have a clue. Just cause they got some training to become qualified as a mental health supporter doesn't mean they are truly capable of understanding everything. For example, I used to use childline so many times when I was younger & every single damn time they ended up making me feel worse. Their opinion of you isn't factual & only what you think of yourself is important. Do you think you're normal, what you're doing is completely fine? Then that most likely is true. Continue to slay the game & as soon as they do something that makes you comfortable, pick yourself up & just know that they're not right. You be the better person & continue to be your definition of a good worker & person. Hang in there !
(Also by the way, they have purposefully made it so that you can't contribute to the chat, if you do say something they'll just find a not so nice remark, express yourself & if they have a problem they can sit and be miserable about it, you lose nothing!)

Be yourself.

sorry, i have a headache right now so i couldnt read your thread fully but be/act as you like/feel comfortable with. really, why care? some day we will all die and everything will be over, enjoy it man.

write in huge junks without paragraphs because I'm defo gonna read it.

It was my birthday this week they all wished me happy birthday on what app. I checked on the day but didn't see any posts or anything. Three days later I checked and they had all wished me happy birthday (from 3 days ago) so Ive only just replied 'thanks, had a good day'. I wasn't sure if it was deemed weirder to post 3 days late than it was to not post anything at all

They have a group on whatsapp which they all talk a lot on I very rarely contribute/ post anything (which they prob think is odd)

From what Ive typed I think there was one missed opportunity where I could have split text into a paragraph, other than this Id say Ive pretty much typed in paragraphs. I may be wrong..

20 years at a sports club must make you very old then (compared to the average user on here), since you're now working in mental health I'm sure you have a lot of wisdom to pass down