Is the marriage contract considered valid if the husband had committed fornication and repented from it, but then re-established contact with other girls without reaching the point of fornication, and then got married with the intention of staying away from sins, while he has doubts about the validity of his repentance?

Your marriage is valid, and your first repentance from fornication is sincere. Your subsequent fall into minor sins (lamam) does not invalidate it. For it is not a condition of repentance that one never falls into the sin again, but rather that one resolves not to return to it. As for the whispers of Satan that trouble your soul, turn away from them and turn towards Allah by performing acts of obedience and shunning acts of disobedience. Praise Him for granting you success in repenting.