If life give you lemons.... Make lemonade. Your situation limits the time you can spend flying. But take advantage of the time away from your flying to really get to grips on the theory and planning (as this seems to be an area you've identified as being an issue). Read your books cover to cover until you really know the stuff - and seek advice about anything that confuses you.
Practice flight planning whilst you're away - get your instructor to email you hypothetical sorties and plan them carefully using the actual weather of the day. Send it back to him/her for review and comment (it may cost you a few beers but it will be worth it).
Make notes after all your lessons and debriefs and read these regularly whilst you're away from flying - it will help you keep your head in the game so that you're not slipping so far back between lessons.
As your proficiency in the theory and planning improves, your stress levels will reduce and your confidence will improve. This will make it easier.
But you need to work at it whilst you're away.
All the best
OH