This is my second marriage. No kids from first marriage. I often find myself thinking of my first Husband and wondering if things would have been different if we had children. Maybe we might still be together. Is it ok for me to feel like this?

What happen had already happen. Doesn't mean you have kids, you may not divorce too. Maybe you divorce few years later instead. Now many people hold back for the kids, but while holding back, did both of you did something to save it and turn it back? If not it's just a matter of time when to settle down and separate from each other. I do quarrels with my husband, but the quarrels don't end without a solution, if a quarrel always end without a solution and just let it go, it'll always repeat. Use it as a lesson, to let the 2nd marriage to be better, there is nothing to feel ashamed of. In fact, many people leave even happier in 2nd marriage (if you found the right one, not the abusive type.)