Gosh. 20/20 is difficult for me to deal with.
I'm reading your post and thinking to myself, why doesn't she just move on? They've been dating for 10 months and the writing is on the wall.
I never want to put myself above anyone and I know I have it easier than some people and worse than others but sheesh! Life gets harder or more complex as you get older. If you're not okay with his actions now, you probably won't be later but later will probably be harder. Take this opportunity in front of you seriously and do whats right for you.