Recently I have been heavily tempted with lustful thoughts. I have tried to resist them when I realise what's happening, however yesterday while tempted I imagined something I ought not have. I am unsure if I gave 'full' consent. I think I may have as I imagined something after being tempted. My priest has said in the past that unless I give 'full' consent it's not a mortal sin. This 'fullness' of consent is where I struggle.

Any advice is appreciated. Anon for obvious reasons.

If you're not certain that you gave full consent, most likely you did not. Many of us go through similar struggles. The best overarching advice that I can give is to use this struggle, and all the difficulties that you encounter in life, to bring you closer to the Lord. 

Generally bad advice here, especially for a person prone to scruples, where there can indeed be "harm done".  Priests often tell scrupulous penintents NOT to confess sins they're not sure about, since they can torment themselves over it.  There are degrees of choosing and some ambiguity about WHAT is chosen.  Often scrupulous individuals confuse feelings of attraction toward beauty with "impure thoughts", even when they're not even primarily sɛҳuąƖ in nature ... so there would be some question of what was chosen and how aware one was of the choice.

Ultimately, yes, but people can be in situations where they can't always get to Confession to a valid/Traditional priest before the next time they have the opportunity to receive Holy Communion, and so they need to make a practical judgment whether they can receive.  There are many chapels where the priest has about 30 minutes to hear Confessions and the lines are long even if you get there early.  Sometimes they'll hear more Confessions after Mass, but that doesn't help someone who would like to receive Holy Communion.