Just checking but have you equipped your characters with materia? Pairing something like Healing materia with the Magic Efficiency one will allow you to heal more often (or MP Absorption paired with damaging magic will refund you a little bit of MP when the damage hits). Using the Speed Up materia also increases how fast your ATB fills which in turn allows you to use abilities more often.

A couple things off the top of my head:

• Barret is a pretty easy character to play since he's ranged and has an ability (Steelskin) that boosts his defense by a lot. Enemies will usually target the character you're controlling so it can be useful to switch to him and use that if someone else is getting their ass beat. Also his Focused Shot ability is really good at staggering enemies.
• Like someone else here said you're kind of meant to switch between the characters often. Also remember to fill out their Folio skill tree thing because you can unlock a lot of really powerful synergy skills that way.
• Always have at least one character with the Assess materia and always cast it on every enemy the first time you fight one. It shows you exactly what strategy you need to beat them (usually in yellow text) and what elements they are weak against. The First Strike materia will also let you cast Assess pretty early in the fight.
• Aerith is a really good. Aside from healing the whole party, she can also put wards on the ground that do things like double cast offensive magic, change her attacks to hitscan-type lasers that build a ton of ATB and stagger (good when you need to heal ASAP) or temporarily stop any enemy that tries to melee attack her. By the end of the game she was my favorite character to play.

Also this isn't really related to the difficulty but one piece of advice I'll give is: don't feel like you need to 100% each map as you get to it. I started off doing that and around ~60% through the game I was getting really burned out. Just do what looks interesting and you'll be fine. After you beat the game you can go back and finish any stuff you missed anyway. Damn, typing up this reply made me want to go back and play the game again lmao.