I suffer also from the same problems than you, really. Many matches i could not close because I felt nervous with my service having not enough spin or variation, or starting to worry about my receive. After, when watching videos, I think this is mainly a mental problem. If we're up 10-7, certainly we did a few good things to get there, including some good serves and receives.
In the recent weeks, of course i'm trying to serve and/or receive better, but now, I mainly focus on getting ready and play the next ball. Doing so, i feel lighter on my feet, and having more reaction time, so I can also stay more calm. If you watch matches from pros, even for them, from time to time, they make some weak serve that gets immediately attacked agressively, but the good players are quick to react and get ready to counter or block that strong receive when it happens.
If you have more reaction time, you can make the right decision. Its not true that you need to attack strong, or even attack every ball, at least at our level, to win. I train a lot with juniors, a few months ago, i felt very often very late when playing with them, and thought that its only legs and physical condition and that i could never compete with them. Well its true i'm not young and cannot be as fit as they are. But a lot of it has to do with replacement and replacement is also a lot about bad habits and the brain more than the legs. So surely my legs are not quicker than 6 months or 1 year ago, but still my body is responding that fraction of second faster WHEN i can be in the right MENTAL state.
As for variation, I was told that i didn't have enough variation as well. I don't have many serves but its also a question of discipline. Have some plan about some ideal percentage for each type of serves, (you can tweak that depending on the opponent), and try to stick to that percentage. You should be aware and not being told by your partner or coach in between sets that you've been serving everything to the BH side for example !
Also executing the same serve, but from a slightly different position (50cm more right or left) or a slightly different toss (higher or lower) is often enough to derail the opponent.