A fellow AWer made a post this morning that reminded me of a conversation I had with my father when I was starting my previous career. He told me about The Rule of Firsts:
"The first day is the hardest, but the next isn't as bad. The first week is the hardest, but the next one is much easier. Then the first month. Then the first year. Focus on getting through that 'first' and you'll be fine."
Same applies to writing. The first MS is a huge learning experience, but your second is a lot easier because you've learned how to create characters, develop a plot, smooth pacing, exercise your voice,etc. And since you've actually completed something, it's not as daunting to do it again. Short stories, flash fiction, writing prompts, etc, all help develop the skills.
This is where tenacity plays a role in writing. Accept that the first manuscript is the hardest and get through it. The first submission to an agent/publisher (and subsequently the first rejection) is also painful, yet necessary hills to climb. Few writers make it big on their first try. Maybe you'll get published and maybe you won't, but quitting before you overcome a hurdle guarantees the latter. Be tenacious and remember that each milestone is a huge step forward to the end goal. All you need is to get past that first.