*boop*
This is a mix of the absolute worst advice and some of the best. To address that in order: revenge is better never served, because all it does is serve to perpetuate a cycle of wrongs that never get righted. If he gave her an eye for an eye, soon enough she'd see how it is and things would end. It'd be him going out of his way to hurt her, which is wrong to do.
Likewise, missing someone is absolutely
not weakness. He obviously cares greatly about her, and while I agree he shouldn't let it consume him and his life, it should be noted that these fearful thoughts are
not being weak, they are part of being an emotionally driven being. Chicks aren't ravenous predators picking off the weakest from the herd, and a dude that shows feelings and concerns is more of what the world needs. Most of my friends are women, and I have been in numerous relationships and even have found out about a few chicks crushing on me, not a single one of them thinks I'm weak for freely admitting my feelings, no matter how sad or scared they are. In fact, his admission of his feelings is far more admirable than the fool that sweeps them away and lies about them being there to start. Nobody is perfect, and everyone has weaknesses, and if someone leaves you for feeling you're not "strong enough," they should go and reevaluate their views on people as a whole. I mean, how shallow can one be to just bail on another because they're not "strong enough?" He has a flaw that he has to work through, and so does everyone else. She has been lacking communication despite being supposedly romantically involved with him, so that is a mistake just as much as thinking the worst due to that.
Each individual is as important as the last, but the individual must be around in order to be that individual. So let me agree with Teiko here: don't freak out and definitely don't ruin your own life for anyone else. If someone is doing more harm than good to you, you have a right to leave, and that is a judgment call. For example, physical abuse, manipulating them to feel some way or another, constant derogatory comments, verbal and emotional abuse, greed and other forms of unreasonable selfishness are all things you should be leaving them for. And since everyone is as important as the last, that means nobody is more or less important. So a fair balance of selfishness and selflessness is important for achieving a healthy lifestyle.
Now as for what you should do, Senjiku? I suggest you consider a few things. Life is crazy sometimes, and people will want to get away from it all from time to time. It's important to give people space when they ask for it. Try having a mature conversation with her (no freaking out or getting extra personal about it) about how her sudden, extended departures make you feel, and ask her all about what causes her to want to flee so maybe it can quell such negative thoughts in the future. Make sure you keep avenues open so you can talk to friends and spend time with them when she's gone, and if you can't have a friend around for every day she's gone, find something else to distract yourself with, like going out for a drive, reading, playing games or whatever it is you enjoy. It's important to learn how to rely on yourself as well, but never be scared to share how you feel in situations like these, because bottling things up inside can be far more dangerous than someone reacting poorly to sharing them.
And dude, I have had all of those same thoughts you listed before. I have since overcome them, myself. Yes, it still feels like shit when you don't hear from someone you care about for a little while, but it's not at all the end of the world.