You know, this is going to sound strange, but it may just be the 300 barrier that's keeping you back.
Time and again posters complain about being just one pound away from big milestones. It's so frustrating! I've been there. See all my mini goals in my signature? I stuck one pound away from each and every one of them!! I think it's possible we put stress on ourselves about it unintentionally. We can't help being excited to get there, but our bodies know. We can never fool our bodies.
Advice though? Stick with it.
That's my best advice. Always before my problem with weight loss was not losing weight. I always lost it, but very slowly given the intensity I was putting in with both diet and exercise. The problem is I always gave up. So this time instead of committing to a poundage goal, I made a goal of sticking through this for a year. I made it through one year and was/am so proud of myself! I lost 75 pounds in one year's time just because I didn't give up.
Actually busting through a plateau? I've always had a few tricks up my sleeve to try.
Increase the calories, decrease the calories, cycle the calories (some high days, some low days), increase the veggies, change the time of day you eat each meal, increase the intensity of the workout, add weight training if you aren't doing it and increase it if you are, add more activity to your day by getting in 5 minute walks here and there or doing squats during commercial breaks.
Basically, make some kind of change. Our bodies get really efficient with whatever good we are giving it and sometimes they fight us.
And just so you know, I have/had PCOS.
It's not impossible to lose weight, but I do think those of us who have it take just a bit longer to reach our goals.