Two things here:
1) You have demonstrated to yourself that (almost) nothing in this test is beyond you: if you're thinking clearly, the problem is doable. Use this conviction: if a problem seems unsolvable at first glance, stop everything, look upwards, take a deep breath, and tell yourself "No GMAT question is unsolvable. This question does have an easy solution method hiding somewhere: I just need to find it. I can do this." Then take another look at the question with a clear mind, and solve. It's the same as opening a stubborn jar of pickles: twist the lid as hard as you think you possibly can, then dig in and twist till it opens.
Even experts get tunnel vision sometimes, and we don't always see the solution immediately. The difference is that we've solved enough GMAT questions (and tough ones, too) to know that if it's in the test, there HAS to be a way - it's not rocket science. This conviction, and the resulting confidence, is usually what it takes to tide you over.
2) The problem may stem from the fact that you're looking at the question, but not at the big picture, but I'll need more info on your timing to say if that is the case. If you let go of a tough question but end up finishing the section ahead of time, then you have lost out: you could've spent some time standing up to the question, but you didn't know it.
Thus, when you next find a tough question, take a look at the number of questions remaining and at the time: the two should hold an approximate 1 to 2 ratio. (e.g., at Q10 you have 27 Questions to go, the clock should show ~54 minutes).
If you're ahead of time (e.g.the time shows 56-60 minutes), it means that you've rushed ahead, and you can afford to relax now - you have just provided yourself with the hardcore evidence that there's no rush, and you can slow down and look at the question with a clear mind. Not that this is ideal - if you've rushed ahead of time, you have probably made some careless mistakes along the way - but at least you know that you have the time to deal with tough cookie on your plate right now.
If you're behind time, then it might be a good idea to let go of the question.
Check your pacing periodically (every 5 questions is a good rule of thumb), and work in a relaxed but methodical manner until that point at which you see that you're falling behind your time plan. Then, and only then, is it time for drastic measures such as letting go of a question early. The idea is to reach the final 37th question with 1.5-2 minutes - no more, no less.
