Is it so hard to be committed in a relationship?

I've never had an issue being committed in my relationships. Unfortunately I cannot say the same about some of my prior partners.

I think if you partner requires faithfulness and you cannot give that to them, then just find yourself an open relationship or a polyamory relationship. It's not too difficult to analyze your personal likes/dislikes and seek out a relationship situation that's ideal for you.

Even if you're in a relationship and suddenly realize that it's not what you want, it's always better to communicate that message to your partner.