This could be your test. 300mg/ml is fairly high. It takes more slovents to hold the hormone in the oil. Plus the more hormone per ml, the more likely it will be painful. There are a couple ways you can reduce the pain. One way would be to find some sterile oil, which can be found at web sites that sell solvents and chems used for home brewing gear. I would try cutting each ml of gear with 1ml of oil. Then see how it feels. From there, you can try to reduce the amount of sterile oil you use and see how much you can tollerate. Another option would be, if you want to add another compound, something that isn't painful, like EQ, this will help to reduce the pain. 

Of course, using less gear per shot will reduce the pain too. 

Some guys can tollerate higher dosed gear than others. In the future, you might be better off using a standard dosed gear. For test cyp and e, 200-250mg is standard. For EQ and deca, 200mg. For tren ace, test prop and mast, 100mg is normal

Last, larger muscles such as glutes are going to be a safer bet with painful gear. There is more room for the oil to disperse.