I do the same too when I make a nice hit in the morning just after the opening. I often am hesitant to go back in and give it all back. It is probably a confidence issue since if you are 100% confidence in your approach then you would only go back in on strong signals and make more money over the long run (i.e. some days you wold give back profits and most days you would add to profits.)
It makes sense but it is still hard to implement. However, I am more comfortable taking the rest of the day off if I hit it nicely in the morning. What I do is I watch the charts and make notations of when I would enter a position or not and then watch it in real time. So even though I do not go back in with real money, I am still studying the chart and working on my entries and exits. Some of these paper trades are profitable meaning I would have made more money and some turn against me meaning I would have lost (not necessarily 1:1 of course). But even though I am just doing it mentally, the exercise still helps me work on my entries and exits while preserving my day's profits.
Over time, the more you do that, the more confidence you will have in your approach and then you WILL start adding more positions in the afternoon or throughout the day and should still see net positive days. I do not mind sitting out at times as it is teaching me some discipline ( too much overtrading in the past).
So I no longer cringe when I see great entries in the PM after I made some money in the morning. I know that if I keep studying them and keep gaining consistency then I will start taking those other entries and overall hopefully raise my net winnings.
EDIT: For example I missed an entry now on the er2 breakout lower but that is ok since it is easier to study it with no cash in lol. It would have rattled me a bit as it would have moved a point against me before moving lower so I prefer seeing it now than jumping in on it and sweating it out.... so even if you decide not to trade anymore for the day, it does not mean the work is done. I still study the charts.