I'm in a similar boat, playing low/mid d3 tennis. Just to give some perspective, I have ok serves (95-105 flat/slice, generally can place), pretty good racquet head speed on groundies, but have low shot tolerance and have only started developing somewhat good volleys. Been playing for 5 years or so, would probably be upper 3.5/lowest of 4.0.
What I would suggest is to find a set of simple, achievable goals that you stick to for an extended period of match time, for example 3 games. Could be something like "deep to backhand" or even "2 feet over the net," but it has to be concrete and achievable. This is difficult if new to match play, since it's easy to lose focus and abandon rational decisions after a long point, especially one you lost. When playing extremely consistent players, I sometimes start trying to go for baseline winners or serve and volley on second serves for the first time in my life, etc. If you have it, suppress that urge.