After 4 years of not being in a relationship why does your ex boyfriend still take you on trips and keeps you involved in his children's life but hasn't seen anyone since your relationship?
I am assuming that there is a sexual relationship going on between the two of you. Because he has children he's probably afraid to commit to a relationship because of the children. It doesn't matter what the reason, stop letting him abuse you in his selfish manner. Tell him to commit or you are walking away from it all and not more trips! If he refuses to commit then you are only providing him with his sexual needs and perhaps being a built-in babysitter.