This is highly personal and I hate giving advice, but since you ask, here it goes.
I am going to focus on "how to pick between ideas" and not on "how to feel confident with my own work".
One technique that might work is to think of yourself as the client. Set a timeline, brief yourself and set a budget. Stick to it. It will be like that in the real world. True, there will be some room for negotiation, but at the end you will need to meet the requirements of the projects. There is always room for improvement. But there is not always necessarily time, money or freedom to do so.
Don't go changing the briefing. Be nice to yourself. You will hate when a client does that to you.
Don't be too nice to yourself. Be true to the budget and timeline. Clients will be extremely irritated if you don't do so. An irritated client is one that does not come back or threatens with not paying. At the beginning, promising a timeline will be hard, because you will not know how long certain things take you. It takes practice. Practice with yourself. Try to estimate how long something will take. But be true to your promised timeline and try to see how you can stick to it without asking for an extension. Change things in your internal workflow to meet your promise.
If you have to break the contract, make sure that it is absolutely necessary. If you need more time make sure you have a good supporting reason for it. If you need more money, the same. If you need to change the briefing make sure it is absolutely necessary, not just for the joy of trying something else.
Train yourself to pick the best choice under those restrictions. It is not the best absolute choice. It will never be because you live in time. After you have made the choice you keep living, thinking, improving. The environment also changes. But for that time, it was the best you could have chosen. Stick to that choice. Learn for the next case and move on.
Remember you are usually designing for somebody else. You are creating a functional product up to specs defined by the market, the industry and the client. It is not art. It has a huge business and functional side.
Finally, don't get burnt out. Burnt out minds don't work. Give yourself a break. Go for ice cream. Walk the dog. Watch Doctor Who. Sleep on ideas. Take long showers. Research has proven that the subconscious processing of problems has a big impact in creativity. It does not come out of the blue, but out of the back of your mind.
