How should I tell my kids that I'm remarrying?

I have 2 children, the eldest is 18 years old while the youngest is 13. I have recently proposed to my Filipina girlfriend. We met through an international dating social event in Cebu, Philippines. She's really a blessing to me. My expectations when I was in the tour wasn't high to begin with but when I met her, it was definitely something unexpected.

My sons doesn't know her and I have never mentioned that I'm seeing someone. There's a reason why... I wasn't completely sure that things would go well between us and I didn't want to introduce someone to them when it's not certain yet that things would work out.

This awkward and worrying situation is my fault. It kinda slipped my mind on letting them know as to what's going on with me when things have started to progress at a certain stage. It also doesn't change the fact that I don't know how to bring this up to my two teenage sons. I really do need help. How can I tell them? Or rather, how should I tell them?

P.S. I'm 44 and my now fiance is 26 years old.