Blow jobs

So I give my boyfriend a blowjob every once in a while. I honestly feel like every time I do, it isn't good enough for him. I feel like I never pleasure him the way he wants to be pleasured. I end up getting upset about it cause I feel like I'm doing it the wrong way for him and he constantly reassuring me with saying, "You're perfect. You're doing nothing wrong. Everything you do feels great to me." And sometimes my mouth starts hurting and stuff and then I lose the feeling of being turned on. So then when he's about to come, I get tired and he loses it. He's never mean about it. He's constantly saying "I love you, you'll always be perfect for me. There's nothing wrong with you." but then he starts to feel upset because I'm upset and there's just so many upset feelings over one blowjob. Any advice?