My first birth: My contractions were painful like stubbing your toe is - a shock through your body, then fading between contractions. I thought I was managing fine, but they're pretty relentless and it does get more and more intense. It then became like really awful bowel-emptying food poisoning, by the end it just washed over me and I mooed away while in a birthing pool and puffing on the G&A. This was described afterwards by my DM as a 'pain-free' birth: it wasn't.
My second birth: I did a breathing class beforehand, and knew what to expect. I knew that the food-poisoning feeling was just all my muscles working to open my cervix, which helped. I also knew that I'd get a baby at the end of it (I was a little surprised first time round!), and that I'd have to 'lose' myself in it to cope. I tried to get into the feeling I had in the birth pool, so I didn't fight it at all.
Finally I breathed big big in and out breaths through each contraction, and soon figured out each would last no more than 4 breaths, so I felt in control. I also had a speedy labour, the result being that I honestly didn't find it as painful in any way as the first time. It just felt like my body was opening up - yes some pain but like if you run hard, like your body is working hard.
I used (the same) tens machine for both births - 1st birth I had it up to 8 by the time I went into the pool, 2nd time I never had it above 4.
So even if you had a painful first experience, it might not be like that the second time.