One thing that helps me maintain an interest in playing guitar, is that I recognize that no matter how good I might get, there will always be room for improvement. Kudos to you for your desire to continually develop your skill!
In answer to your question
Should I play on my own, listen to covers, learn from ultimate guitar or try joining some group?
My recommendation is to do any or all of the above, as long as it keeps you interested and excited to continue to learn and improve. Anything you do on guitar will help you improve. But some things will motivate you more than others. Things you enjoy will be more likely to keep you interested in spending time with your instrument.
Spend more time doing things you enjoy than things you dread. You want playing time to mostly be something you look forward to. But some hard work is a necessary ingredient of improvement. However - the drills, practice and other intentional hard work will be more fun if you have a goal you are working towards that promises a sense of satisfaction and pride once accomplished.
I find that playing with other musicians provides both inspiration and an opportunity to learn from them. I have developed a number of close musician friends who I met at events that attract musicians (open mic nights, song writing seminars, music sessions, open jams, house concerts, music festivals, etc.). Sometimes I will just get together with a friend and jam, or I will participate in open jams that are organized by local musicians I have connected with.
Another thing that really inspired me to accelerate my improvement trajectory was joining (actually forming in my case) a band. Like you, I never thought I was good enough to be in a band. But I continued to get great feedback after performances at open mics from other musicians who even invited me to join their band.
Then I was "forced" into starting my own band after sitting in on a few numbers with another local band who introduced me to the manager of the venue. I was called later that week to ask if "my band" could perform the next Friday night. So I scrambled to put together a band and create a set list that all the musicians knew - and the rest was history.
So don't ever think you are not good enough to be in a band. You may be surprised to learn that there are bands who could benefit from what you might have to offer. Don't hold out for top dollar for your first band placement - offer your services for the opportunity to learn and have fun. Maybe you could be a backup guitarists for times when the regular guitarists can't make a gig.
Playing in a band provides a whole new level of inspiration because other band members are constantly pushing you to learn new material to expand the repertoire of the band. And practicing with the other musicians in the band who must all learn to maintain a consistent tempo and work together as a cohesive musical entity - will help you become a better musician.
It's been said that "you play better golf when you play with better golfers". The same concept applies to musicians. If you are lucky enough to be the least talented musician in the band (like me), the other musicians will both inspire you and help you improve. Being surrounded on stage and at band practice with superior talent, gives you more motivation to step up your own game.
Perhaps you can make being in a band a goal that will help provide motivation to continually improve. Keep learning to play new material. Try to learn to play popular songs from memory. Look for ads at music stores and in local newspapers and online (including Craig's List in the US) for bands looking to add a guitarists. Go to open mics and talk to other musicians.
If you can't find an existing band that needs your services, don't be afraid to start your own band. If you believe that you could get a good band booked at local music venues that pay - you will find musicians who will play in your band.
Keep reaching for a higher level of musicianship. Create goals that motivate you to work towards a tangible or intrinsic reward. Most important of all - keep it fun and enjoy the journey! Rock on bro.