why my girlfriend and i are having so little sex?

[Sentip]

so my girlfriend and i have been dating for just over 2 years now, and our relationship has been going/ growing very well. we basically live together all but maybe 1 day a week. we've talked about our future and even marriage. i love her deeply, we are best friends, and love spending time together. problem is we don't have very much sex, hardly any lately. maybe once a month. now i should note we are only 24. we've brought it up before, and both know there is a sexual deficit, brought it up quite a few times, but always kind of make an excuse like, "we're just tired" "we've both been working allot lately" and as that might be the truth at times, i feel like there's more to it, like a lack of attraction? i do still feel attracted to her , but there's just not the feeling of " man i really want her right now" and i understand that the honeymoon phase passes, and we've both put on a few pounds since getting together(nothing excessive), but it really feels like we are falling farther and farther apart from the sexual need that most couples have for each other... so much so that when we do have sex, it feels like it's almost awkward... or forced? i don't know. but that's why I'm asking on here, lol. so just looking for some advice, maybe I'm not preforming well enough for her, and that pushed us apart, maybe there's just not enough physical attraction? but i would appreciate any advice because i love her more than anything, and want to be with no one but her, but how do we save our sexual relationship?