I know that a lot of people don't like Franklin Veaux, but I find this article
helpful in a general sense.
Not a lot of specific practical advice, though. The advice seems to be, don't just endure it or you'll be miserable, but don't forbid it either. Allow it, but then ask for reassurance, and retrain your brain so that men aren't seen a threat because you trust that your partner will make decisions that take your well-being into account, that she loves you, etc.
Ultimately, I think a way to get over it will be for you to get over the fear of not being the best. If you can reach a point where you can think "if the guy is better in bed than me, stronger, better looking, and his dick is better... so what? Good for her."
For that, you need to know what's being triggered, what you're afraid of, and follow the train of thoughts.
You're worried, for instance, that a guy might have a bigger penis. Why? What bad outcome would follow? It itself, it's not scary, unlike you have the largest penis in the world some guys will be better-endowed. Why is it scary? Do you fear it would make you less valuable in her mind? Do you fear she might think "why did I bother with him? He has a small penis!"
Fear don't have to be rational to be crippling, so be willing to admit that you're afraid of something that, when you say it out loud, sounds ridiculous.
Whoever she dates, he or she will be better than you in some ways and worse in others. You're the sum of everything. Is the size of the other guy's penis scarier to you than whether he shares an interest with her you don't share, and they can talk about it for hours? Or the idea that, say, they are a similar size and she can wear his clothes?
These examples are random. But penis size is random too. It's just been ingrained that bigger is better, which depends on the taste of the woman, and the skill of the man. And I think that has a lot to do with why you don't have a problem with female lovers: they don't have a penis at all.
Although, they actually might have several. And if you're so worried about size, well, the penises they might have are probably bigger than yours, because they rarely sell average-sized toys. And if all else fails, they have fists, too.