We found the visa process pretty stressful as well. Back when we applied we were really worried about our finances, so I ended up staying in the US for 4 months with DH working in the UK before we thought we were ready to apply for our spousal visa. Looking back we probably went a bit overboard and could've applied earlier and not spent so much time apart, but at the time we were so scared of being denied and not being allowed to live in the same country together. It didn't help that besides each other, our families, and this board no one we met seemed to realise how difficult the process was, so we ended up doing a lot of worrying with each other rather than to other people who didn't really understand why we were so scared.
Like vchpa, we thought of the worst case scenario, and that actually made us feel loads better. At first the worst case was not being together, and that was really hard to deal with. But we knew that the hardest applications would be to get me to the UK or DH to the US, so we decided that if my application to the UK were denied then we'd move to nearly any other country in the EU, which would be super easy to move to since DH could use his EU treaty rights and I could go over as his spouse. Some countries like Germany and France and Ireland really didn't seem like that bad choices to live in for a couple years. Then we could either reapply for a spousal visa to the UK once we were more settled, or wait a few years until DH could qualify to apply as a British citizen living abroad in the EU, which was a simpler and cheaper process. We even picked out our second choice country and when I felt stressed I'd look up social groups and cool things in that city that made the second choice seem not so bad. And the main thing was that we knew that if my visa were denied then we could still be together, and that helped so much in making the application process less stressful.
We never would've told an ECO about our backup plan because we've always wanted to live in the UK together and we wouldn't have wanted to give anyone reason to doubt that. It was just a much worse second case scenario that was pretty much a coping mechanism, but it helped, because we knew that no matter what we'd be together. And luckily my visa was issued and we didn't have to use our backup plan.
We went through tons of crazy stuff due to immigration compared to what our friends have had to go through in their relationships; the amount of stress and money we've invested in visas, and time spent apart. And so much of it happened really early on when we'd only been together for a few months or a year. But we got through it all, with both of us stressed for what seems like years, and I think it actually made our relationship stronger than it would've been if it had been easier because we knew we wanted to be together and could get through anything, and then we ended up proving it. We just got our ILR in the summer and even we were surprised at how much of a weight we felt taken off of both of our shoulders, even after being here two years on a spousal visa.
Just remember that all this visa stuff is temporary. I know it's scary thinking of a government not giving you permission to live in the same country as your fiance, but if you're on this board and were knowledgeable enough to send in a complete visa application (bearing in mind that I don't know anything about your specific case) I think it's really unlikely it would be denied. If it is, there are backup plans that would let you two still be together, and eventually move to the UK. And once you get through it all your relationship will hopefully be stronger as well.
It's so easy to let the visa stuff worry you so much that it's hard to relax and think of other things, but really now that it's submitted just try everything you can to take your mind off of it, especially if you're the one who's more worried and kind of bringing it to the table, even if you're trying not to. Try to spend time with your DH talking about things that don't involve the visa or immigration and that aren't stressful. Do all the things on the lists of activities for LDRs, like date nights watching films together with your fiance or playing board games online, to give yourselves an activity to do together that'll hopefully distract both of you enough for you to just have a good time together.