Upvote:5
Nobody can tell you how difficult it will be for you to get into Scotland. You will either be admitted to the UK or sent to the US. The probability of either outcome depends on factors that we do not know, some of which we cannot know, such as the mood of the immigration officer who interviews you on arrival in the UK and on your ability to present a coherent explanation of your circumstances.
It makes far more sense for your girlfriend to apply for her visa in Spain and for you to travel directly from Spain to the US.