Speaking in tones can be a daunting prospect when you start out. You are trying to do too many things at once, getting the tones right, getting the pronunciation right, and getting the meaning right, and you will stall. It is much better to work in small steps, and focus on just one or the other. Start with small speaking drills and you will find you are able to cope. You will find your mouth can get used to how to form the sounds, and your ear can get used to how they should sound.
One idea for a speaking drill is to focus on two-word chunks, rather than complete sentences. Take “hǎo chī” or “hěn máng”, say them, and pay attention to the tones as you go. Go slowly, as this isn’t a race, and say them a few times. Then stop and listen to the recording. Then say them a few more times. Repeat the process. By going back and forwards like this you will start to connect what you are hearing with what you are saying.
If you try and say long sentences too early in the process you will end up going too fast. Your tones will be poor or even non-existent. If this happens, shorten what you are saying. Cut it down to two or three syllables and work on those until you can do them, then add in the next syllable. In this way you can build the sentence up without letting the tones drop.
You don’t need to spend a long time on your speaking drills. You could easily spend fifteen minutes on just five pieces of language. You could spend the first three minutes listening to each of them carefully. Then spend the next three minutes saying each of them slowly, trying to exaggerate the shape of the tones, and leaving a little pause between each attempt. In the following three minutes you could try and say them without listening to the recording first, to see how well you have learned them.
Finally, spend the last three minutes going back to the recording, and practising some more. Some days you will find your voice doesn’t want to behave. The tones will be running together and you won’t be able to cope. When this happens don’t push on and try and do more. Instead, slow down even further. Stretch the syllables out a little longer and just let the shape of the tones become very clear in your mind. Then go back to your speaking drill. You will find you can do it much better.