Like xxxDogmanxxx said.
In your nasus vid
At lvl 1 you took a lot of free damage. Yes CS is important but you're vs a range champ don't go for cs that's far out. At level 4 she hit you and you walked into your wave. She took almost more damage than what she did to you. This is what you want to do against aggressive players like that early on. Also in that time frame after when she was under 25% mana you let her do free damage to you. Hit her back homie hit that e then drop the w into an auto ->q then back off. Don't just walk out, punish them.
Later on if you still cant stack like lvl 6-7 just let her take the tower and farm between 2nd tower/destroyed outer tower. This way if she comes that far in to hit you and cs your jungle can gank easy. This will help you scale easier with less danger to you. All you want to do on nasus is farm to the point where you got 2-3 items and 300 stacks before 25-28 mins then start port ganking. If your team is being shitter then you keep pushing my boy. Shove the lane and create pressure while you scale. This will give your team time to farm cs while enemy is busy dealing with you.
Dont worry about roaming so much on nasus before 25 mins. Just farm and stack your q as much as you can.
For your Kat game
You shouldnt have pushed that much early. If you noticed that Zed was very stiff and looked like he didnt know what he was doing. Let em come to you man. Just Q harass and at 3 just keep repeating Q W E to your minion and try to stick behind your minions. You had good trades. If you had let him out to the middle of the lane you could have killed him at 3 after baiting out his shadow. If you push them into their turret so much then you cant kill em early without dying to tower.
In higher elo the enemy jungle would have killed you a lot of times because of how deep you were in for most of the early lane. Just be a bit more patient with trying to hit and kill the enemy. Dont want to push em into their tower too much unless you're backing or roaming. Its very dangerous when you do that. You are too open to a gank or taking free tower shots then getting killed like you did at 6 mins into the vid. Your jungle also cant gank for you early like that(especially in low elo because most junglers are clueless and wont dive safely)
Buy a pink on 2nd back or 1st back and place in the big bush to the bot side of mid (if red side) and top big bush(if blue side). For your yellow place it a bit further out like the single bush in middle of river(face check bush). Gives better vision and earlier warning. You were pretty deep in and that bush line ward isnt going to help you much. So ward accordingly to where you plan to be.
You could have ganked top a good amount of times in laning phase since the akali was doing the same thing as you(pushing in super close to enemy turret).
Teamfight
Your team fight at 15 mins could have gone better. When zed marked you, you should have jumped to your galio if possible or blitz. Be close to your team. Remember kat is squish and you cant fight 3 ppl on your own when you're not fed. You ended up getting one kill and splitting off from your team. Xin would have died anyways because of his really bad positioning. Ashe, blitz, galio, and you could have fared off better if you guys stuck together. Low elo and stuff so dont mind that much as its hard to team fight in bronze/silver/gold due to teammates. It end up being a 4-4 but I think your team would have won that if you stuck close to them.
At 19 mins when akali and WW came to take inhib you should have let them and then killed the rest of wave before it reached nexus tower. All you had to do was just walk around in that area. Your team got wiped and you died which is really bad. Just play safe in situations like this in the future. Unless you can totally destroy them, there is no point of going in to that.
At 21 mins, I know how enticing that zed looked but there was no way you could have gotten to him. Dont do these kind of chases. It just ends up wasting time. You could have cleaned that whole wave then cleared wolves/raptors. Keep your mind on getting farm when its looking shitty like that and your team isnt grouped.
At 25 mins in that fight you used your only escape on an enemy that was in your range. Its just little things like this that a lot of ppl make mistakes on in low elo.
That game could have been winnable but you didnt let zed out of his safe zone much to land some kills. Other than that I think it wasnt bad. Your CS is decent for that elo but could be better with more focus on rotation.
The best advice
I can give you is Pick 2 roles and only 2 no more. For example, I main mid and only mid. Then I have jungle as my secondary if I dont get mid. After that pick 2 champs only for each role. One is your main and the other is the one you play if you its banned or you dont feel comfortable picking into something. Nothing more. Practice these 2 champs in normal games for at least 20-30 games each and get a good feel. After that take it to ranked, learn, and get more focused on the points people have listed in this thread. Bronze-gold is mostly knowing the games basics. Like keeping up CS, rotations, and when to roam. After that you want to get better mechanically on your main champ to climb further. Once you learn the basics man you will get out of bronze-silver quick. That doesn't mean you should ignore getting better at your main champ because you always want to be improving on that as well as the games basics.
Post more vids and stuff if you need more help!
Im struggling with similar things but my mind is full dum this past week so im Tiltimus Prime atm. Good luck to the both of us.