Always welcome @outlander
I think you hit the nail on the head to where you are most likely to gain success socialising - those with similar interests in horses, nursing, art, studying - there are many art and craft/writing meet up groups; and courses to check out there my friend. There are many different avenues. Sounds like the group the group you are in presently is cliquey - that is always more difficult. See if you can find a group or a course where most people are meeting for the first time or taking in many new comers. That would be much easier. Even if you try 20 - one is bound to be good. Don't give up.
Yes, I have had many patches in my life where I have found it hard to enjoy life. I witnessed my daughter's near fatal attempt. She was in despair due to an an abusive relationship; she looks back now sees what a big mistake it would of been. She was lucky there was no brain damage or permanent injuries.
I know what it is to lose zest for life. But it can come back. And It can get better. As we go forward in life we mellow some and appreciate things more. The course of our natural life is short anyway, it goes fast. Best make the most of it opposed to the other option that offers nothing
I really do believe once you find your niche and purpose in the world (which takes time, a path of self discovery with lows and highs), and start having fun with people your age - discovering what it is like to connect with a special few, your zest will return. Your self esteem and worth will increase. It did for me. And the world will seem less bleak and scary. In time my friend.....I definitely connect better in real life than online as people get to know the real me and deep connections form.
How's your health? Do you laugh often? ( it's the best medicine). Being healthy makes "a big difference to our moods". When I am not I get tired and my zest can go to zip - so looking after our health needs (exercise, healthy eating, proper rountine sleep, relaxation and interests etc) all helps get the mind and body in order. A few things to work on and you will start to feel a bit better. Keeping the vision positive hanging onto hope. There is always some good in life to savour 
With your interests you would have done well living in the country (if you don't already). I use to own a farm and had a friends horses on it (and minatures ones as well, they were cute), plus other livestock. And arts and crafts, gardening were the rage. A lot of nurses lived on properties ther . I loved my time living there. But now am enjoying a sea change.
I am off to my volunteer work tomorrow which I am looking forward too. Keep stepping out of your comfort zone my friend and try different things. Don't worry what others will think of you - just take it real slow getting to know people before you trust and the big one - "just be yourself" with people you meet. Those that matter will appreciate you as they get to know you - the others don't matter. People like it when we are ourselves and natural - that's all you have to be and friends will be made eventually. Sweet dreams @outlander 😴💞