The sauce has a slightly bitter edge to it but if it is overwhelmingly so something is not right. I have thought about a few factors that might contribute. Firstly, it might be the type of lemon – some are more bitter than others. From experience I learnt that if you cook lemon (even the juice!) for too long or hard it will turn bitter, so shorten the cooking time or put them in at a later stage. Lastly, maybe try less lemon next time, maybe half or a quarter of lemon?

I hope it helps!