Cymbalta (Duloxetine) is known as an SNRI (selective Serotonin AND Norepinephrine Reuptake Inhibitor). It is similar to a drug known as Effexor.
As you stated in your question, phentermine has a mechanism of action that is very similar to amphetamine products which is why it is classified as a controlled substance. There unfortunately is not another prescription product classified as an antidepressant that will give you that immediate boosting feeling that phentermine provides. Having said that, there are some antidepressants that work on the same neurotransmitters which may have a better effect for you.
Phentermine, like amphetamines mainly increases the release of two main neurotransmitters, norepinephrine and dopamine. This results in CNS stimulation among other things. This CNS stimulation is what is typically believed to cause that mood boost you are feeling.
Celexa (citalopram) is known as an SSRI and only works on serotonin. Therapy with SSRIs can take a long time to achieve desirable results. I don't know how long you have been taking the medication but most practitioners want to give it about 4-8 weeks to re-evaluate their patients taken those medications. If one SSRI isn't working there are many options available including trying another SSRI or switching drug classes altogether.
There are two classes of drugs that may be of particular interest to you since they work on similar neurotransmitters that phentermine affects. These two classes are SNRIs (Serotonin-norepinephrine reuptake inhibitors) and drugs that affect dopamine such as Wellbutrin. Both classes of drugs are very commonly prescribed and may be a good option for you. Examples of drugs in the SNRI class include Cymbalta (Duloxetine) and Effexor (Venlafaxine). Wellbutrin is an example of a drug that affects dopamine.
I would certainly talk to your provider about a possible switch in medication. There very often is a therapy out there that can help those who do not respond to SSRI drugs like Celexa.