Yep - with proper ones, you can't really hear anything live. There are some good squishy foam ones too - where they expand in the ear canal, plus you can even make real ones yourself, buying the putty from ebay! two part stuff. Mix the two balls together, squeeze into a cone and get somebody to press it into your ear, and the folds - then trim and drill a hole and glue in your headphone drivers. Real ones are super comfy, but you must have the mould made with your mouth open if you sing, otherwise when you open you mouth, your ear canal changes shape and breaks the seal. With proper fitting ones, the volume in your ears can be lower than real life stage levels.
HOWEVER - if you have IEMs, you really need a personal mix, because if the mix is wrong, you cannot sing or play. You can play and sing with a less ideal mix by just moving your head or body, but this does not work with IEMs. If you cannot hear, you are stuffed. I once played an entire song on my bass in E when the idiot on sound sent electric guitar, not bass into my ears, so I just had to play with no bass at all. My mistake was the song was in Eb, not E and I never knew.
If you have never had a proper monitor mix and have got used to the audience mix you will have compensated. There is a snag with your system. What does the PA mix have in it? Everything, or just the things that are not already loud? In a smaller venue, even with the kit and bass coming up on faders, it's very common for the faders to be off - they're loud enough already. Mixing from the stage means you cannot tell how it sounds in the audience. Probably fine, but probably bass and drums heavy, or light. I have a show arriving Thursday who will be mixing from the wings, in a 1400 seat theatre venue, with a stage 12m wide and 11m deep. They don't usually do theatres but smaller clubs and pubs. I'm very worried about their sound. The bands who tour IEMs always have perfect sound in their ears, and the audience get perfect sound too - however, they're very different mixes. I bet during the soundcheck you wander into the audience area to check the sound is OK there, but you have no way of compensating for a loud audience, or one in the winter wearing thicker clothing that soaks up the top end. We used to pay our sound op the same as the band members got in our tribute, because he was vital to the sound we wanted. Audiences rarely mention sound unless it's very bad, so the absence of complaints doesn't mean it was good, just not bad. I've heard some of my band's festival mixes where we were not in control and many are dire because the audience are getting far too much bass, and very little harmony vocals because the op doesn't understand the music!
