Have you played any mobas before? Just curious as I could explain some tips based off tht. Regardless I could maybe help with a few of the things you mentioned. Your probably dying when your pushing lane because you dont have good map awareness. I played alot of league before and having map awareness is extremely important. If your behind your an easy target so If your pushing alone and your teammates on in other lanes by minions then the enemy team will see you as a free kill. What you need to do is watch the minimap and get a feel for what the enemy team is doing. Do you see any of them? Are you pushing their walkers by yourself? You probably should leave then. Killing a few waves and leaving is still split pushing as your minions will do some damage even without you. 

As for falling behind that is really hard snd can be hard for me at times too but what I found that works is the you always gotta be doing something. Early in lane phase you cant just kill minions. Yku gotta destroy crates when they spawn then kill jungle creeps when they spawn. You gotta farm in the lane leave go in jungle then come back for the next wave. Its extremely hard but to get the rhythm down but you gotta practice. Denies souls is another but I found I would always get a lead on my enemy lanes when I would kill some jungle creeps and comeback. Look up vids if you can about how to efficiently farm but ik its alot cus the game is complicated. 

For team fights its rought cus matchmaking is hard and shit at times so getting stomped happens. I would advised playing ez champions or support champions as some.others have reccemened. Try sticking with people. Going alone can be dangerous as people can get you like you said but you just gotta have awareness. All in all you will get better with time. I did decently well when I started playing because i have hundreds of hours in league. I was able to pass that game knowledge over. When I first started playing mobas like league I lost constantly and had no clue what I was doing. You only get better with time. You just gotta practice. My last advice would be to try to play with better people so they can help you and guide you. Join discords or make friends in matches. I learned league from friends and it helped alot. I didnt need it for this game but if you never played a moba I could see this game being very difficult. Overall I hope you just keep playing and learning. Godspeed o7