Lack of Orgasms. And it's hella annoying.

I had an IUD implanted about three weeks ago now and right after my boyfriend and I could start having sec again, we did and it was great! But every time we've had sex this week, I haven't been able to orgasm. It's so frustrating!! I will admit things have been a little stressful with grad school and running a company, and I have a poor knowledge/idea of how sex should be. I put pressure on myself and think I have to cum. I start trying too hard and it just kills it all together. I also have a mental clock that starts going off because I feel like I'm taking to long. I know he doesn't mind for obvious reasons, but he talks dirty and says he wants me to cum, which I know is in no way pressure, but then I think, "Yea I want that too. Let me try harder," and I'm back to square one.