I try to give each of my children attention and love but, for example, my 2 year old gets jealous whenever her older sister (5) is trying to talk to me. She starts yelling loudly to prevent the conversation from excluding her. Then the 5 year old gets frustrated and cries and the one year old feels the upset and starts crying also. Right now I am alone with the kids all day during the week and can't afford extra help. I'm worried about making mistakes with my middle child. How can I help my 2 year old? How can I keep my sanity and give each child what they need? I'm feeling overwhelmed!
First I might look for ways to carve out some personal time with each child. Perhaps when the 2 year old is napping or if the 5 year old goes to bed in the evening a little later than the 2 year old you could give each of them special attention at bedtime. Another idea would be to hold the 2 year on your lap as you work with the 5 year old so that she feels connected to you even as you give your attention to the 5 year old. This may or may not work considering the 1 year old as well.
If you have to be with both of the ages at one time you might make it a game. You could have a timer and say that for 3 minutes the 5 year old has your lap and then the 2 year old has the next 3 minutes. Or for 1 minute the 5 year old can talk and then the 2 year old gets one minute. Maybe the 2 year old can hold the timer too or a comfort object that might help her wait. Even 2 year olds get the concept of fairness and definitely the 5 year old will so using a timer might help. Also if the 5 year old goes to school then that is perfect for extra time with the 2 year old.
Be creative and use your intuition and do anything that comes to mind to level the attention you give to both at the same time. For example, give hugs and kisses (like a game) to each when you are all together so they both see that you love each equally.
It's also helpful to remember you really can't make mistakes and children are the most resilient creatures and have more understanding than we think they do. You are doing your best and you can even talk to both about the dilemma you are having with reading or playing with them both at the same time. They get it on some level appropriate to their age and may have some ideas on how to share your attention.
I had 4 children and now 4 grandchildren so I know not to spend time thinking about the mistakes or bad days but just stay as patient and positive as any one day allows! Remember you are a good mom for your kids just because you love them so much! If you didn't, you wouldn't be worried about this.
You mentioned that the 2 year old feels excluded. Just taking a moment to put ourselves in her place it's not hard to see why she might resort to shouting! At 2 she is still somewhat a baby when you think about it and not fully independent, though beginning to experiment with that. Having a younger sibling in the mix means in some ways she isn't the baby any more and that can be tough on both mom and child. I agree with Hassi's suggestion to find even little ways to give each child their special time with you, even if it's just a few minutes.
Another idea would be to work on learning some calming strategies together as a family. I have some videos of playful breathing techniques that you might like to try. This might help you as well! If we're feeling overwhelmed, the kids will pick up on this and as you mentioned the 1 year old picking up on the upset of the 5 year old. Probably the best thing you can do is find ways (late at night, early in the morning maybe) to have even 5 or 10 minutes to yourself to get centered or do something that helps you feel relaxed and peaceful. Even if this feeling doesn't last you can build on this over time and it will be a resource for you in those stressful moments to be able to stay calm when everyone else is crying!
As for what to do in the moment, I think pausing the conversation with the 5 year old to fully acknowledge the 2 year old is important even if she is interrupting. Interrupting is kind of an advanced concept for her age to understand and respect. Many adults still don't get it! Pausing to give her some sense of attention or connection (even with touch or holding her in your lap as Hassi suggests) to reassure her while listening to the 5 year old might be helpful. Remember, she's expressing a need the best way she knows how, which is really uncomfortable for everyone. She needs some help learning some new skills to use.