Spaceplane piloting is basically trying to get SAS to settle down on the pitch angle you want to climb at and then not touching the controls too much. Fine adjustment mode can be helpful to avoid jerky manoeuvres.

Personally though, I just used an autopilot mod such as: Kramax Autopilot Continued [forum.kerbalspaceprogram.com]. (I've been using the original version of this mod because until I went looking for a link I didn't realize anyone had taken over maintaining it. The original actually still seams to work fine in KSP 1.9).

Good luck with your spaceplane attempts. It's just like everything else in Kerbal, to figure out how to do it successfully you first have to figure out as many way to not do it successfully as you can ;D.

I tried it with a few different fuel tanks, because the purpose of using this vessel was to go into orbit to refuel the space station. I honestly think I'm just a bad SSTO driver. I've tried many different ones and creating my own using all the knowledge gained from YT videos. No successful results to date. Thanks for looking into this and following up with me.

I ran a couple of test on this craft and I was not able to reproduce the issue with it having difficulty accelerating. I usually test with an autopilot mod for consistency, but this time I made sure to test in a fully stock game.

Th only guess I have at this point is the payload is adding drag to the craft even when with the bay closed. I use ore tanks as a test payload to find the max load, but they make for a fairly streamlined payload. When actually using the craft to lift more complex payloads in game I don't usually get close to its max load so any drag would be offset by the higher thrust to weight ratio. So maybe a very heavy high drag payload would slow it down?

Sorry I don't really have any other ideas why it might not be working properly.

Hmm, that is odd as the CR-7s were powerful enough to accelerate in a shallow climb when I made this craft. But I've not had a chance to play lately, so I will try and test it again when I get a chance. I'll see if I made a mistake in the flight profile or something changed in the game. Only other guess I have is extra drag from pitching up either due to an off centre payload mass or abrupt control input, but I not being able to break 350m/s in level flight suggest something is very wrong with the design.

Thank you for taking the time to let me know about the problem.

I've tried every which of way to fly your craft. I've tried staying low and building up speed, I've tried following your instructions, and I've tried climbing up, nose diving to build up speed, and then climbing out. Bottom line is I can't get this ship over an average of 330-350m/s or higher than about 10,000m in altitude before it just gives up.