Is this repentance sincere and acceptable to God, despite remembering the sin (kissing the fiancée with lust) while feeling the desire and pleasure to repeat it, yet adhering to the vow not to return to it?

Repentance from sin is valid if its conditions are met: abstaining from the sin, regretting it, and resolving not to return to it. If the sin involves the right of a creature, then it is obligatory to fulfill it or seek the owner's forgiveness. The thoughts that may cross your mind after repentance do not harm, as long as you repel them and they do not lead to committing something forbidden. We hope your repentance is accepted, for having a good opinion of Allah is required, and He, glorified be He, accepts the repentance of His servants.