A longer break in playing is always frustrating, but - like a lot of problems - it can also be seen as an opportunity, if viewed in the right light.
The feeling of picking up an instrument for the first time is something that is hard to replicate. Most of us pick up playing habits over the years and they aren't always good ones. Getting rid of them is hard, as I've learned on more than one occasion.
You're starting with something of a clean slate and here's a chance to take your playing further than before. You no doubt remember a lot of the basic skills - and so do your fingers - but getting your playing in shape is going to take some work. Take this opportunity to really work on the fundamentals.
Finger mobility and strength, conditioning, stretching, technique — all these things you'll probably find are a bit rusty. That's good! Get a good technique book or two and devote some time every day to working through it. Try to really pay attention to the details. It will take some time for your hands to get back in shape, so fast technical playing will probably be out of the question in the beginning. Focus on slow, deliberate practice and working things out step by step. You might find that you'll emerge a better player. (The same ideas apply to other body parts if your instrument uses them!)
Of course, all practice and no play makes Jack a very boring musician, so don't forget to set some time aside for music. What did you use to play? Revisit some of the tunes that were a major part of your repertoire in the past. Start with the easiest ones first (maybe some you stopped playing, because they were no longer challenging — now's the time to reacquaint yourself with them) and move on to the more difficult ones as you regain confidence in your playing. What are you listening to now? No doubt, you've picked up some new music along the way. Try to get some new tunes under your belt, if you feel up to it.
You have the opportunity to reinvent yourself as a musician — taking the best of what you used to play and bringing in new inspirations and your development as a person. Just take your time and allow your playing to gradually reach a satisfactory.
No different than teaching a music student, except that in this case the student is you.