There are many many times where you had an opportunity to get a killing grab and instead went for a non guaranteed or weak smash.
Vs sheik you can dthrow chain grab even with just popo if you follow DI to about 50%. When they are at 100% and you can get a grab just jab then charge a smash with nana and then dthrow or upthrow into that smash for easy guaranteed KOs. You iceblock in ineffective and unsafe ways, in general you only want to do it to force them to recover at odd angles or to force them to approach, however sheik can approach ice blocks very safely anyways so its not a very useful move vs her.
As far as using it to force awkward recoveries they need to be very far off and you generally would desync and have nana iceblock so you can get the smash edgeguard or the ledge hog. Against Sheik though you dont need to do this because her recovery is so easy to read, just wait the proper amount of time, grab the ledge then time a regular stand up off the edge(by tiling towards the stage) and forward smash her off if she tried to recover high to fall back down to the ledge or grab her and do a reverse forward smash to send her back off the stage. (By grabbing charging a forward smash with nana and doing an up throw to reverse the direction).
Other than that you have a bunch of common problems associated with being newer at the game, asside from playing tons and tons of games or practicing regularly vs computers there really arent good ways to improve your general fluidity in my opinion. So keep practicing and have as much fun as you can.
GL in future matches,
Binx