If you can orgasm by stimulating your clit, then it's a bit inaccurate to say you can't orgasm!
Not all women can orgasm from penetration -- I *can*, but I have to be reeeeeeally horny -- so when I was a young floozy having only occasional sex, I could always have a penetrative orgasm. However, with my partner, because we have lots & lots of sex, penetrative orgasms are rare for me... but it's not an issue for us! I come every time we have sex by stimulating my clit while we fuck, either with my hand, his hand or a vibrator.
As for why you might be struggling recently, the pressure you're under to climax (from your partner and from yourself) is definitely a factor. The more you try, the more difficult it will become. There's no way to guarantee a penetrative orgasm, but there is one essential ingredient -- relaxation. If you're worried about it happening, it probably won't!
It might be worth talking to your partner and explaining to him that all women climax differently, and the vast majority of women can't climax from penetration alone. Let him know he's not a failure in any way, because it's not possible to fail at sex as long as you are both enjoying yourselves! :) Hope that's helpful!