How to keep up the pace buring a blow job?

Here is one of the best books in the industry on the subject. I have made a link to a Review by Airen Wolf: The Ultimate Guide to Fellatio by Violet Blue, Second edition 2010. In this book there are may great techniques which will help you avoid the issues that you have noted here. I hope this helps.

I'd suggest you sitting or laying down comfortably with a pillow behind you and having your guy be the one to put it in your mouth, whether he's on his knees or standing up. It would relieve the pressure on your body. It's what I did after surgery when I couldn't move much and it worked great.

Stretch it sweetie! Your mouth and tongue are muscles. You may feel silly but just start playing with your mouth and tongue more, it's the easiest first step, be aware that you have it, move it around more, make silly face while you're driving in the car alone. This will activate the muscles and make them slowly stronger.

Then, don't be afraid to add in your hand and give that jaw and neck a break. You can make a blow job sexier with extras, it isn't just about your mouth. That's just one of the components when blowing him away

Mostly though, experiment! Find new positions and things that work for you. Ask him what he wants different. Listen, and don't take it personally. The more your experiment, the more perfect you'll customize his oral experience!