I reckon you should'nt be in ANY relationship at the moment let alone being engaged, an engagement is a commitment.
You ''DON'T'' give your mobile number to another guy unless you have serious doubts about your current boyfriend, i feel sorry for your fella as you have portrait him as a lovely guy and yet you are doing this behind his back, either tell him or do the decent thing and leave him, as the only one getting hurt here is your boyfriend, and another question, how is he the '' MAN OF YOUR DREAMS'' when you only chatted to him at the bar {briefly i assume}.
I think at 21 you see this fella {maybe goodlooking} and compared him looks wise to your current boyfriend, which if true is shallow minded, you are playing a dangerous game with the txting, and my betting is you WILL get caught out by your boyfriend, and he'll dump you, if you want to PLAY AROUND well stay single {no commitments} you owe that much to your current bf