A bit of backstory: I broke up with my first boyfriend because while I was gone at an engineering camp I developed feelings for another guy. My thought process was "If I can develop feelings for somebody other than my boyfriend, he obviously isn't the one".
So for the past almost year and a half I've been dating the man I've been convinced I'll marry since I met him. I recently came to the realization that our relationship isn't as perfect as I'd thought, and the only reason we haven't fought more is because I'm way too tolerant. He chews, which I get is a personal thing and I haven't asked that he stop, only that he stop when he knows he's gonna be with me, and I've known that for awhile. The problem is instead of coming clean, he repeatedly lied to my face about it, saying that it was his friends that do it. He even told me I wasn't allowed to open his glove box, and got mad at me when I tried to open it to stash away some spare napkins (that's where he kept the full spit-bottles apparently). And that just one of the few issues. We aren't compatible in our sex-life (we both want different things), he's so locked in his walls that half the time he can't talk about anything but the military and alcohol, which by consequence means he's not very close to my mom (who is my very best friend) and he's by far the least romantic guy I've ever met. I mean in the sense that for valentine's day he made plans to go watch the hockey game with some buddies, and would've only spent an hour and a half with me had I not gotten upset about it. And he often puts me into situations that trigger my social anxiety BAD, then whether inadvertently or not makes me feel bad because I had a panic attack and refused to suddenly go talk to every one of his family members. This isn't the end of it though. My best friend, who happens to be male, is everything my boyfriend isn't. He's a bit of a liar I'll admit but he's romantic, our sexual preferences line up almost perfectly, he's capable of full-length deep conversations, and just to spite me even more he and my mother are friends. They don't just get along, they are friends. The have a poke-war going on Facebook and frequently message each other to catch up. My mother absolutely loves him. Did I mention that I come from a marine family, and he's a marine, whereas my boyfriend is in the guard? By all accounts, my best friend is better matched for me than my boyfriend... but I love my boyfriend. With all our stupid inside jokes (i.e. "Pug Lord". We made a fake religion that "worships" a pug that looks incredibly derpy on a poster in Target) and whatnot... but I've had an on and off crush on my best friend since I met him... What should I do? I don't want to lose the man I love, but...
It sounds to me like you aren't letting yourself attach sufficiently to begin with. So with that the case, of course you're too easily 'stealable'.
Also, what's this about 'been convinced you'd marry since you met him'? Do you do that with cars, as well?... just sit in the drivers seat, admiring the smell of the upholstery, etc., and saying, 'I'LL TAKE IT!'?
And how is anyone supposed to take up position as your number 1 if your mum - your original, albeit platonic, love - is still sat in it? Are you and your mum too close, still? Did dad eff off when you were young?
If it's not under-assertiveness then... people who care a great deal about something tend not to be all that tolerant. Ironic though it might seem, where a man doesn't like a woman who complains non-stop and/or the wrong way (e.g. unnecessarily antagonistically), he DOES take reassurance in the fact OF complaining. If you're not bothered about anything then he knows you're not bothered about him and yours and his future... what it'll look and feel like ergo what, now, you're fighting to pre-establish. So it's perfectly possible that he can't be arsed to do x, y, z if he senses you aren't all that into HIM, particularly if he's a reactive rather than proactive type in this arena. He might swing between this apathy and trying to see if you can be upset, though (flick - OW! - "Whaaaah!" = she *does* care!). So I doubt it's inadvertent despite it's probably under his own conscious radar (possibly not, possibly a mixture between unwitting and deliberate).
"My best friend, who happens to be male"
THERE IT IS! So there are TWO 'lovers' already sat in that seat, aren't there! How on earth is bf supposed to feed you as creates emotional dependence when you're already stuffed to the gills? Hardly surprising that you describe your love for your boyfriend as centred around - to the point of hinging on - superficials. That, to me, makes it sound as if you love him as a FRIEND, not a soulmate.
I suspect it's NOT that you don't want to lose 'the man you (cough!) love'. I think you just don't want to find yourself single in between relationships.
Why not? Plus, how is bbf supposed to decide to plonk himself into a chair that has bf (and mum) already sat in it, same as, how is bf supposed to commit to that chair when  it's only half vacant and  he can see another man sniffing around ready to leap into it or be invited into it?
You're being a commitmentphobe. Do you realise that? 30%-committing to everyone and not 90% to anyone. Any *one*. Bar your mum who's position is guaranteed - neither man stands a chance. And if, for example, your boyfriend can't commit to you because you're not committing to him, then how on earth do you expect him to work any wooing magic on you to GET you attaching? Catch 22. Self-created.
You'll have to choose a pot and actually piss in it. One, two, three, JUMP! Oh, wait... A decision is a commitment, isn't it. Awwww... ;-)