If you're decent at Melee, it should have taught you the skills you need to overcome these simple problems: put forth some reads, react appropriately, and use mindgames. Overly defensive play in Smash 4 will get you punished if your opponent is good.
Sheik should have no problem with most of this stuff:
1. If your opponent's holding shield and you feel the need to approach, just dash grab or pivot grab if you read a sidestep. Otherwise let them come to you, it isn't hard. If they keep holding shield like that, it's going to break. If they shield because you've left the ground, make the jump empty - it's a solid mindgame and a mostly lagless landing into a grab or a forward tilt is a good choice vs this. Sheik has good followups from her grabs, too. If I remember correctly, nair is safe on shield.
2. You're playing Sheik, you don't have to deal with this. Throw some needles and knock them off, run in for a ledge trump, bair. Or, throw needles and pressure their shield and force an approach. Honestly I've never seen that sort of thing outside of For Glory, and if you're judging your ability based on that hot mess of hell, stop.
3. Read.
4. Sidestep, jump, airdodge, jab (no grab armor in 4), grab first.
5. Look into ledge trumping. I can't believe I'm saying this, but watch Zero play some Sheik.
Honestly a lot of this just sounds like you're unfamiliar with the mechanics of the game. I went back and played Melee a few weeks ago for the first time since Brawl came out and it felt awful, so I can see how you might need a little time to get used to how things work now vs how they worked long ago. Search My Smash Corner or Beefy Smash Doods on youtube and watch some of their quality learning material, it'll help you with basics if you're lacking them.
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