You should be honored for striving to give your kid the complete set - even though it is hard as heck on you.
Be encouraged, you are not alone. Even if the dad is around, we still have to be the father when our kids are with us.
If the father is around, let him be the father figure also, allow him to take his role, and encourage your kids to see him as such - remember they don't have to know what you know about that person, and whether we like it or not, that dude is or will be their "hero" at some point, let's not tamper with that.
Be strong for your kids, be strong for you.
HAPPY FATHER'S DAY TO YOU! YOU ARE A FATHER ALSO!!!
I am praying for you, and give your burden to God.
Blessings! love your kids today.