I want to make my nose wider (alar base increase) again and open up the nostrils.

Seems your nose needs a very very complex revision procedure to, essentially, perform an augmentation, lengthening or expansion rhinoplasty due to a previously too aggressive structural reduction affecting not only the nostrils, but also the nasal lenght, rotation, caudal septum, etc.All the former is feasible and treatable, requires top notch skills in structural revisions, grafting, experience, artistry and plenty of patient, however can be done in the right hands.Regarding the nostrils reconstruction... if it is required only a slight augmentation of the nostrils there are methods to reinforce them and thus expand them a bit; if the shortfall of tissue might be larger then additional fresh tissue has to be supplied by means of, first step, a composite grafting or, second step, a local or regional flap to provide better aesthetic quality tissue.The forehead flap is an option, true, but not the only one and actually not the first choice for your case; I'd rather rule it out as first instance and I'd be more thinking about some local flaps like the nasolabial flaps, turnover flaps (revolving doors) and others, but to tell you a better opinion we need to see your case.Regarding the narrowing of the nasal valve that can be fixed by structural expansion, this means additional and new cartilaginous grafting.See the link below to find cases of my own practice very similar to yours which I had the opportunity to operate successfully on, sharing a lot of common features with the technical problem you have posted.
If you wish better grounded opinion well lit, focused and standard images have to be assessed: frontal, both lateral and both oblique views, also underneath the nostrils.
Feel free to request any additional information from me.

Sometimes we add cartilage to improve the structure. Othertimes we just move the nasal skin and that can correct the problem.  Photos would help considerably in helping to determine exactly what is best for you.

You do not need a forehead flap for this.  In fact the treatment can be quite simple and it involves using a composite graft from the ear to add width to the alar base.