Joining a drama club or doing drama. SO much fun and helps a bunch, unless you don't like drama then I would suggest assertiveness training or positive thinking. Spend more time with friends and write yourself lists about what you're good at or good things you've done that you're proud of and read it to yourself often. A great start is asking for help which you've done so, awesome job

Thanks :) I think I'm too old for drama club. I'll just keep trying to reconnect with old friends and keep plugging away at dragon boat.

Do something you never would think you'd have the courage to do. Once done, no matter the result you can take pride in yourself for giving it a shot and facing down that fear that was holding you back.

I would suggest you your username.

I AGREE WITH @parisqueen THEATRE IS THE MOST UPLIFTING CHEERFUL AND SUPPORTIVE GROUP OF PEOPLE EVER YOU ARE NEVER TOO OLD LOOK UP LOCAL COMMUNITY THEATRES AND WORKSHOPS FOR ADULTS

Play an instrument then perform in public yo.

"Fake it 'til you make it" doesn't sound like sage advice but it really does work! Pretending to be confident actually makes you more confident over time. And positive affirmations help improve self-esteem; keep telling yourself "I look amazing," "I'm doing great," "I did a really good job at this thing," etc and you'll start thinking more highly of yourself! If you have to do something that requires a lot of confidence (job interview, etc), go into the bathroom and do a superhero pose (legs apart, hands on hips, head proudly tilted towards the sky) for a few minutes. Just the action of doing this pose will increase your confidence and self-efficacy.

Have bombass eyebrows and eyeliner

Sometimes starting basic and simple can help. Like taking a bit more time with perdonal grooming, or a new outfit. Do some extra cleaning on your living space, your car or bicycle (unless you take a bus or train everywhere.)
Consider community theatre, like the others have mentioned. Acting if you are extroverted, backstage if you are introverted (like props, costuming if you can sew, lighting or sound if you are technically inclined.) I did this for many years, it helped me immensely.

You are never too old for drama my friend, one of my high school teachers does plays and musicals and she's in her late 50's and she's wonderful. It's up to you of course but don't limit your options with what you think you can and can't do, just give it a try.
Anything that involves a community of people all involved in one common goal that they're all passionate about really really helps, so try looking for groups like that.
Everyone else's suggestions are great too, like @unicycle said, positive self talk is important and helps and sometimes changing up your appearance works too.
If you like reading I would suggest some magazines I love called Frankie, Slow and Woman Kind. They're aesthetically pleasing, have little gifts inside, stories and ways to show yourself love.

Everyone I've seen here has made good suggestions. I'll sum up my general thoughts. The things you say to you, when you doubt yourself, put yourself down, or punish yourself? You likely wouldn't say that to a stranger in the same situation. So stop. Don't be nicer to others than to yourself. Stop telling yourself bad things about you. "Fake it till you make it" means that even if you don't feel like it, force yourself to be positive. Love yourself and give yourself permission to fail! You'll make mistakes. We all do. Whatever crushed your self esteem, acknowledge it happened. If there's a lesson there on what to do next time learn it, own up and move on. Any time you fail learn and move on, each failure will bring you closer to success if you don't keep repeating mistakes, but repeat them as much as you need to learn your lesson. Look it all as growing. Force yourself to take care of yourself. Groom, maybe change up your look a little, work out, eat healthier, try new hobbies,learn.

Also try a 'Compliment Book', it's just a little book in which you write down any compliments people have given you or nice things you've read that make you happy

Sorry. Last bit. Depending on what happened it may take time to heal and feel good about yourself. Be patient and keep trying. If you don't get results right away don't give up, but don't be afraid to try different things until the right one hits. Be around positive people who will support you and help keep you positive about you, cut out people who drag you down or make you feel bad about yourself. Get someone you know and trust, or a professional you can talk to about your issues if they persist or are seriously impacting your life. It's a process and there's no "right way" except what works for you. Just know whatever happened has happened. Spend 24/7 working to change yesterday and you likely never will. Spend a few hours a day working to change tomorrow and each day you'll see things get a little better. Good luck.

Blast music when you're home alone. Sing with it. Dance. Laugh at yourself for being silly. Have fun!

I love all of your suggestions. You're all absolutely right about everything.
I was being bullied by my manager and hadn't realised how much it was effecting me until I managed to get out on an assignment. I've been at the new job for two months now (halfway through) and my coworkers keep asking if they can keep me. :) I was at an all staff today and ran into a coworker who left years ago, he told me that they'll have a position opening up and said he'll call me when it does.
It doesn't seem like much but they've been the positive things said to me at work in years. The truth is coming out about my manager which really helps that he's not getting the support from higher ups. I've been back at the gym for the last week and a half and loving feeling strong again. Dragon boat season is going to be kick ass this season!!
Love and thank you for all of your support. You're all amazing people! <3

I'm really happy to hear that! Congrats!!

So glad to know there are positives coming out of this. Also the fact you do Dragon Boating is awesome, I always see them when I row and it's the best to watch looks so much fun. <3