Yes you probably are holding on too tightly, especially seeing that you mention you broke up because he thought it was getting too serious too soon. I fully understand what it's like not to want to let something go because you are afraid, but sometimes it actually works the opposite way to what you want it to.
Don't torture yourself with the "what if's". You don't know they will happen, so try not to dwell on something that may or may not happen. It's not going to help you and it will annoy the crap outta him.
Just let go and let the cards fall where they will. If the relationship was meant to be, then it will survive. If not, then no matter where he is in the world, it wasn't going to anyway. Just realise that no amount of worrying or reassurance will change that. Nothing will.
Keep busy and don't think about negative things. Stay positive. Stay busy. Occupy your time. You will miss him, but it will fly by. When you contact him when he's away, don't bombard him with questions about where he is, what he's doing, who he's talking to. Just ask if he's having a great time, tell him casually what you've been up to and let him know that you miss him and can't wait for his return. The last thing he wants to hear is whiny "i can't live without you" sooky type stuff. He doesn't want to hear that you sulked and worried the whole time he's away.
Make it easy for him. Give him reason to WANT to come back to you - even if you have to act because you are worrying on the inside - he doesn't want to see that. And that's not by being needy or clingy. Your best chance is to be fun and light and treat it as if nothing will change and when he gets back you can pick back up and catch up on all the time you missed out on while he is away. Concentrate on that, his long awaited return, rather than something that you imagine "might" happen.
And for goodness sakes, don't stress about it until it actually happens. It's just a waste of time to consume yourself with thoughts of things over which you have no control, and thoughts that may not even come to fruitition.
Have faith in your relationship. He wouldn't have got back together with you if he didn't think you were very special. Show him how special you can be and show him all he will miss when he is away.