The causes of premature ejaculation are mostly complex and sometimes difficult to ascertain as they can be collectively due to various factors like;
2) 5 HT dysfunction
3) Hyperexcitable ejaculation or hyperarousability
4) Penis hypersensitivity
5) Poor ejaculatory control
6) Infrequent sex
8) Performance stress
One or many factors can play role simultaneously. Urologist's assessment would be able to give the clue and then are dealt appropriately.
Following measures are helpful for premature ejaculation;
1) Application of lidocaine / benzocaine on glans about 20-30 minutes before the intercourse (which you have already tried).
2) Learning of squeeze technique; your physician or urologist can give counsel about this (you learn to squeeze the erect penile shaft before the ejaculatory reflex is stimulated; see the resource also)
3) Medical therapy;
a) clomipramine, phenoxybenzamine are the medication which can be helpful to you.
b) Numerous studies have shown that selective serotonin reuptake inhibitors (SSRIs) and drugs with SSRI-like side effects are safe and effective to treat premature ejaculation. So following medications can help (if one is not helpful, other should be tried);
The other way suggested is the masturbate 3-4 hours before the sex. The ejaculation is significantly delayed than with the sexual act, if you get an erection in 3-4 hours.
4) Female up position; this delays the ejaculation
5) Avoidance of stress: performance stress, again increases the premature ejaculation and aggravate the situation.
6) Talk to your physician about the problem in the presence of your partner and together this can be easily handled.
7) Sex therapy sessions.
Read this resource;
So a combination of treatment together would help.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.