I've never squirted, but I have overcome some similar situations with partners. For your case, if you aren't already comfortable masturbating/squirting alone in a tub, try that first. Then once you're comfortable in that setting, try it with your partner there. If you pee in the tub, no biggie. If you haven't told your partner about your fear of peeing in front of him, do so. Talking about it might make you feel more comfortable overall.

Also, if he's just sitting there with you, I be it'd be awkward for you. I love being watched by my partner now, but sometimes I still feel weird if I'm trying something new, etc. Get him to be entertained by something else, be it some other toy or masturbator, just something so he isn't completely focused on you. That might take the spotlight off of you, and also add some entertainment for you if you like to watch him jerk it.

Two final ideas: 1) Do it in front of a camera (even if the camera isn't on/recording). That might get you used to being in front of something when you squirt. 2) If y'all are in the same apartment/house, have him go do something else (unsuspecting), and don't tell him you're going to try to squirt. Then, when you're close, as casually as you can, call him in to see you. Maybe not having him there the whole time would relieve some of the pressure. (Also, if you don't get to the point where you feel like you'll squirt, he'd never know)

I have a problem with squirting. I think that the problem you're experiencing is the feeling of urinating when you squirt. At least, that's what it feels like for me. I know I feel like I'm going to be embarrassed if I let go and accidentally urinate on him. Just let go and let go of whatever worry or fear is on your mind. Work on it slowly. You'll get there

Nina Hartley's simultaneous orgasms DVD made a really good point that I think is applicable to all sex acts: you should be enjoying yourself, not specifically trying to reach a certain goal. I mean, do you go in to sex thinking, "Okay, I am doing this for the orgasm," or do you go in to it thinking, "Okay, I want to have sex because it feels good"? Same goes for squirting: you don't do it and think, "I just wanna squirt I just wanna squirt I just wanna squirt" the whole time, you just like the way it feels, and actually squirting is an added bonus. Don't worry about whether you will or won't squirt and just enjoy yourself.

If the idea of him watching you is what makes you nervous, keep him occupied during. Ask him to rim you, or have him recline in front of you so you can blow him.

I can't stand it to have my partner just sitting watching me. Suggest you have him sit behind you, holing you close and caressing your boobs, nibbling on your ears, neck or anything he can reach...he should be engaged. And don't just try to squirt...the expectation can be overwhelming and thus disappointing.
During solo play I ejaculate frequently but with my b/f more infrequently. I just try not to think about it and accept what happens.
I am convinced that too many guys who watch their partners masturbating are waiting for the porno outcome in less then 15min!

I would try having lots of sex and orgasm(s) before trying to stimulate your g-spot. I find that mine is much more responsive after I've already had at least 1 orgasm. I actually prefer g-spot stimulation as more of a "finishing off" than the main event. So maybe you all could get together, have fun, and then start playing with your g-spot as like the dessert. That way you'll be naturally be more relaxed.

Everyone's answers are great. If you are stimulating your g-spot with him watching and waiting for a specific outcome, the pressure to perform will work against you. Maybe if he stimulates your clit or sucks a breast or is involved in some way it will seem less like he's watching your perform your solo routine.

I'm with Happy Lady in that it's much easier for me to squirt if I have already had an orgasm or are very close to having one.

I can't squirt unless I have a prior orgasm. Should have included that.

Thanks everyone for your advice. Not all of it is applicable (I just get tired after clitoral orgasms, honestly) but most of it is and I'm going to keep it in mind. I'll report back at some point in the future. Thanks again