How to make it easier to hold bar chords
This type of chord is difficult to hold for some, but there are ways to make it easier.
Three things to note up front:
First, it is okay to write out this style of chord as bar chord. Technically, it's supposed to be written as barre chord. But whether you write it as bar, barr or barre, it all means the same thing and they're all pronounced the same way.
Second, while I am showing an acoustic guitar above (the inexpensive-but-good Jasmine S35, for those interested,) this applies to both acoustic and electric.
Third, I'm going to specifically be focusing on steel-string guitars because that's the kind people have difficulty with when trying to hold down bar chords. An acoustic guitar with nylon strings (such as the Fender MC-1) is by very nature much easier to hold down chords with. But that style of guitar isn't meant to be played with a pick, and the kind people want to know about is the one you can play with a pick, which of course is the steel-string.
Using thin strings
Thinner strings does make it easier to hold down bar chords.
I personally use the next-to-thinnest electric guitar string there is, which is a gauge set from .008 to .038, also known as "extra super light." The only set lighter than that starts with .007, such as this Reverend Willy's set that is .007 to .038. Do I recommend .007? No, I don't. That's too thin, even for me.
Where acoustic guitar strings are concerned, the lightest you can go before the strings start buzzing and flopping all over the place is .010 to .047, such as the D'Addario EXP10 set.
Detuning to Eb
For some people, a simple detune from standard E to E-flat works wonders. The overall string tension is loosened and sometimes that's all you need to do to make it easier to hold down bar chords.
Using a guitar with a shorter scale neck
The standard scale length on most guitars is the Fender standard (even if the guitar is not made by Fender,) 25.5 inches. This is the measurement from the string saddle to the nut and not the entire length of the guitar.
A guitar with a shorter scale length, such as the Epiphone Les Paul with its 24.75 inch scale or the Fender Duo-Sonic with its 24.0 inch scale, will make it easier for you to hold bar chords. A shorter scale length means the overall string tension will be slightly looser compared to using a guitar with a standard scale length.
Lowering the string saddles (if possible)
On most acoustic guitars, there's no way to lower the string saddle mechanically. Your only option there is to physically file it down. But I absolutely do not recommend that unless you're familiar with how to install a replacement saddle. This means knowing how to pull out the old saddle with pliers and then pressing in a new one. That may sound simple but it's almost too easy to seriously screw up your guitar if you've never done it before, so again, I don't recommend it.
With electric guitars, lowering the string height is fortunately easy. It is either done by lowering the entire bridge (like on a Les Paul) or lowering each individual string saddle (like on a Stratocaster or Telecaster.)
You will have to experiment with string height to see what works best for you.
Using a flatter neck bow
This is one that's a little scary for some, because it does require adjustment of the truss rod, and you have to know what you're doing before you do it.
Before continuing, know that this mostly affects just the middle of the neck, as in frets 7 through 12, because that's where the bow is on most guitars.
In other words, if you specifically have difficulty holding bar chords in the 7 through 12 fret range and only there, doing what I'm about to say may help.
If your guitar is properly set up where you can hold chords properly in the fret 1-6 range and the string height is to your liking, but anything after fret 7 gets more difficult, it is most likely true the neck needs a tightening of the truss rod.
What does a tightening of the truss rod do? It will bring the strings closer to the middle of the neck since tightening the rod decreases neck bow, i.e. makes it flatter.
How much should you tighten? Only a small amount. A clockwise 1/4 turn should be enough to bring the strings closer to the middle of the neck to make holding bar chords there easier.
Again, I have to stress to only do this if you specifically have difficulty holding down bar chords in the middle of the neck. If you have difficulty holding down bar chords across the entire neck, try detuning to Eb, try thinner strings, try lowering string height.
"I've tried everything above and I still can't do it"
Follow the advice above and try a guitar that has a short scale neck. I mentioned the Fender Duo-Sonic above, but a very cheap option is the Squier Bullet Mustang. That guitar with an .008 to .038 string set is pretty much as "light and loose" as you can get away with. Set that guitar with low action and a flat bow of the neck and that's as easy as it gets to hold down bar chords where steel-string guitars are concerned.