Fit is THE key.

If you're hurting from fit then clearly you can't put forth everything you have, even more subtle changes to fit can make differences in output even if you aren't hurting.

Start here. If you've grown AND put forth a decent amount of effort training recently your fit numbers HAVE changed. You can play with this yourself if you want, but it might not be a bad investment to go back in and have a re-fit done, especially if you are having fit related pain.

As for the bike itself, IMO, there is a certain pricepoint/equipment level that really can make a large improvement. After that you pay exponentially more for very little added gain (and this is coming from someone with a $4k carbon "wonder-bike"...I assure you that is because it has the "ear-to-ear grin" effect, not because it makes me a significantly faster rider than say a $1k aluminum and 105 level bike would).

I don't know enough about the Marin to comment meaningfully on it specifically. Something to think about, if a re-fit will get you comfortable again on the frame and you haven't grown up a whole size, may be replacing a few components. New wheels and tires or maybe a GOOD set of clip-on aerobars if you plan on doing more than the passing time trial. You may not need a whole new bike to feel like you have a whole new bike.