Often when I am tempted by a bad thought or a desire for a future lawful pleasure (at least in reality if it were the future it would be lawful at that time), I do make some effort to banish it when I first realise what is happening, 

This is good.

but even after 1 second it returns and continues 

This is normal.  This is when you gotta start praying and go do something to distract/focus your mind on something else.

and it sometimes takes me a while to realise again that my mind is capture by this.

This is normal.  Some temptations are quick; some last a long time and won't go away.

It's very frustrating as I am unsure if I give full consent or not, because sometimes there is some degree of pleasure but am I unsure if that pleasure is also a trick. 

A temptation by it's very nature has some degree of pleasure associated with it.  If it wasn't enticing, it wouldn't be tempting.  Fr Remler's book tells us that in the area of temptations of the flesh, most of the time, there is a degree of pleasure that is associated with the temptation.  Because our body is warped by Original Sin.  


Feelings of arousal which are due to temptations are not sinful, IF YOU DON'T consent.  (Assuming you didn't expose yourself to an occasion of sin.  If you did, that would be a sin).  We can't fully control our body and the pleasure/enticement of a temptation isn't a sin.  You have to reject the enticement, how ever long it lasts, and you avoid sin and gain merit for winning the fight.

Fr Remler's book is simply fantastic.  It's cheap.  Buy it.  

Ave María, Purísima:

I would advise you to consult a validly ordained priest with solid training and relate him the details under the seal of confession. But just to emphasize what others have said about doubts on whether somebody has given consent to a bad thought or not, here's what Saint Alphonsus said:

And, for the consolation of timid and scrupulous souls, I will here state that, according to the common opinion of theologians, when a soul that fears God and hates sin is in doubt whether she gave consent to a bad thought, she is not bound, as long as she is not certain of having given consent, to confess it: for it is then morally certain that she has not consented to it. Had she really fallen into grievous sin she would have no doubt about it; for mortal sin is so horrible a monster, that it is impossible for him who fears God to admit it into the soul without his knowledge.

Observe that St. Alphonsus is referring to God-fearing souls, so you should not be presumptuous about it. Read the entire sermon and check with a priest.


VivaJesús

EDIT: What I personally do when assaulted by a bad thought is to pray a Hail Mary and "go in spirit to the foot of the Cross in Mount Calvary and hold on to it" - paraphrasing St. Francis de Sales in his Introduction to the Devout Life.