You question is based on a premise that doesn't operate. Which is "I have to feel good to manifest a desire." That simply isn't true. I can understand how that idea can sometimes be taken up though after reading some things from this site and listening to Abraham-Hicks etc.

You are manifesting all the time. You manifest what your vibration is, regardless of how that vibration makes you feel.

When you worry about something, you are projecting yourself in your mind to a future reality where what you do not desire has already occurred, and thus you give off that vibration, which attracts the thing you do not desire towards you.

Your feelings are guidance about where you are heading, and not the "creator" themselves. (That statement isn't 100% accurate, but in this case I think it will help with clarity) Thus, if you feel good you are heading towards things you desire. But equally true, if you feel bad you are heading towards things you don't want. In both cases your energy is flowing - and so in both cases you are creating

Simply put: You manifest all the time, whether you feel good or bad.

As Liam pointed out we are always manifesting whether we are aware of it or not. The majority of people are manifesting unconsciously and have not even heard of the Law of Attraction and thus blame unwanted manifestations on circumstances or other people.

The feeling is a key component in the manifestation process and usually determines what we manifest. When you're not feeling good, it's almost impossible to hold your attention on something positive that you want because your vibration is not in alignment with or on the same frequency as the thing that you want. Your vibration is resonating on a lower frequency so your attention will be on things vibrating at that same frequency - usually things you don't want to manifest or experience.

The trick is to find one little thought that improves how you feel and milk it for all it's worth - in other words give that one thought as much attention as you can until you begin to feel a little better. This will begin to raise your vibration a little and in doing so will begin to attract other thoughts of the same nature. As these new, better feeling thoughts, begin to seep into your consciousness you'll begin to start feeling a little better and the process will get easier and things will begin to change.

It often takes quite a bit of mental work and refocusing but the more you practice, the easier it will get. Good luck!