I'm trying to beat the game in hard difficulty. I need advices from experienced players.

Some info:
I'm not very interested in psionics.
I choose Navy.

If it's your first time, why are you playing it on hard? You don't get achievements or anything.

I normally don't replay games, and I wanted a challenge ( I usually select easy or normal ). I don't care for this stuff. Thanks man!

I don't mean to be too cheeky, im quite light hearted I swear. But if you're asking for advice you might aswell play it on normal or easy. Believe me the game can be a challenge on normal.

IMO Normal in SS2 is like hard in other games.

When it comes to SS2, this is a mistake. I can almost guarantee that you won't see anywhere close to everything on a single run through. SS2 is such a relatively short game, and has such great replay value due to the RPG system, that it's silly not to play it at least two or three times.

It's not uncommon. Most old FPS games are painfully easy on normal due to improved control setups

I think you mean most modern FPS games are painfully easy due to devolved control setups.

What I mean is using mouse-look and WASD in many older FPS games makes them much easier than they once were.

Not many veterans here. Maybe you're right, but the whole point is having fun, and I'm liking the challenge. The game being short doesn't change anything to me. If I really like the game, I'll probably replay (I spent lots of hours in Fo3, and I'm certain I'll play it again with different character). If I don't like it, I won't, I'm not masochist. I disagree that that is a mistake lol.

Hitpoints are fewer on harder difficulties. Worry about evading attacks over tanking damage. Put a few points into agility and the only typical attacks you won't be able to evade are the hitscan shotgun hybrid attacks. Endurance upgrades don't really do too much for you so save your modules there.

Standard weapons is the most efficient weapons track by far. If you worry about running out of ammo, get a couple of points in Energy weapons and carry an energy pistol or two for backup.

This is probably too late for you at this stage, but the OS upgrades can make or break your character. Sharpshooter is good for extra ranged weapons damage, there's also the one which opens up the second implant slot - the one which reduces replicator costs is also useful as combined with hacking can further reduce nanite costs.

Beyond that - save often, keep a stash of med-hypos in your inventory (and hotkeyed to a convenient key!) and try and hunt down the QBR chamber or a working surgical bed as early on as possible (if you're willing to give up an OS upgrade, there's one called Spatially Aware which fills in the whole map automatically except for the last couple of levels).