God, I hate it when people ask others they have never even met online questions like this, but I just can't help but seek others' opinions on this. Basically, I'm still crushed from the death of a girlfriend about four years ago... but I've realized that loneliness is what has stopped me from healing this far.
Here's where htings get tricky. I plan on joining the U.S.M.C. next year if things don't work out with my current career, which means if I pursued a relationship despite my emotional B.S. I don't know how long it would last. I have serious trust issues... I don't know if I could leave a girlfriend behind or not. Any advice or speculation would be greatly appreciated.
Love takes time.

Don't get down on yourself for asking strangers for advice. That's how bartenders got the rep for being great folks to talk to. They're impartial, as we are, and there's no onus on you because we don't know you at all. Neat and clean! LOL

Your question is very interesting. First, my sympathy on the loss of your girlfriend. That was a very traumatic event, and you will grieve for however long it takes. There are no rules about that. Don't wallow--don't use it as an excuse for not moving forward--but allow yourself your sadness. Heck! I'm still sad at times about a lost love who died years ago myself! I can certainly relate. But it's important not to build a wall of sadness between you and your future just because it's easier than moving past the grief.

That said, and given that you also plan on joining the USMC, I'd warn against trying to start a serious relationship. Not only is it not fair to you and bound to bring up your trust issues if you have to be gone for periods, but it's not fair to the girl you might become involved with. But that doesn't rule out dating and fun times with female friends. It sounds as if you need to broaden your social circle to include some women who are just nice to be around and who might go to events with you without expecting more than that.

You might consider taking some sort of class or joining a group or club that isn't strictly male-oriented. The gym is a fine place to start. So is adult night school. Classes on things like investing, photography, bird-watching, genealogy... those are the sorts of things that attract both sexes. Pick an interest (let's at least make it something that will be fun for you, too) and see what you can find at your local high school or college.

My heart goes out to you. I hope you find a way to move ahead and quench the loneliness.